Emission occurs when an element or compound releases energy as either particles (such as electrons or ions) or electromagnetic radiation (such as photons). This process often results from energy changes within atoms or molecules, including electron transitions between energy levels or atomic/molecular vibrations. Emission can occur across a range of wavelengths, including visible light, and is typically triggered by heating or other forms of excitation.
Energy Changes in Atoms/Molecules: Atoms and molecules absorb and release energy, causing changes in electron positions or molecular vibrations. This energy is often emitted as electromagnetic radiation.
Electron Transitions: Electrons jump between specific energy levels within an atom. When they absorb energy, they move to higher levels; when they return to lower levels, they release energy as photons.
Molecular Vibrations: In molecules, atoms vibrate within chemical bonds. When energy is absorbed, these vibrations increase, and the energy can be emitted as electromagnetic radiation, often in the infrared spectrum.
Atomic vibrations refer to the back-and-forth movements of individual atoms around their fixed positions. This is often discussed in solid-state physics, where atoms in a solid are arranged in a regular pattern and can oscillate slightly while remaining in place overall.
An elementary particle ( fundamental particle) is the most basic unit of matter that is not composed of smaller particles. These particles are considered the building blocks of everything in the universe.
In the context of electromagnetism, there is only one fundamental particle, the photon, which acts as the force carrier, transmitting the electromagnetic force and carrying energy and momentum.
One way photons are created and destroyed is through subatomic processes within atoms and molecules.
These processes often involve interactions between fundamental particles governed by the strong nuclear force, which binds the building blocks of atoms (protons and neutrons) together.
Remember that when photons are created within atoms and molecules through interactions like electron transitions and interactions with the strong nuclear force they produce light.
Other light-producing processes (light sources) include blackbody radiation (incandescent light bulb), nuclear fusion (sunlight), annihilation (gamma rays) and high-energy phenomena (supernovae).
Electron spin is an intrinsic property of electrons, along with their mass and charge. Spin is not a classical rotation. It’s a quantum property and shouldn’t be interpreted literally as spinning. It is quantized, meaning it can only have certain discrete values.
Quantized Nature: The spin of an electron is always quantized, meaning it can only take on specific values. Electrons have a spin quantum number s=1/2, meaning they can exist in one of two possible spin states:
“Spin-up” (+1/2+1/2+1/2)
“Spin-down” (−1/2-1/2−1/2)
Magnetic Moment: Due to its spin, the electron generates a tiny magnetic field (magnetic moment), which interacts with external magnetic fields. This property is exploited in techniques like MRI and electron spin resonance (ESR).
Pauli Exclusion Principle: Electron spin is crucial for the Pauli Exclusion Principle, which states that no two electrons in an atom can have the same set of quantum numbers. This is why two electrons in the same atomic orbital must have opposite spins.
Intrinsic Property: Unlike orbital angular momentum, which depends on the electron’s motion around the nucleus, spin is intrinsic to the electron itself. It’s as if the electron has an inherent angular momentum that’s always present.
Electron mass is a fundamental property of electrons, representing the intrinsic amount of matter they possess. It is a measure of the electron’s resistance to acceleration.
Atomic Structure: The mass of electrons contributes to the overall mass of atoms and influences the behaviour of electrons within the atom.
The arrangement of electrons in orbitals and their energy levels are partly determined by their mass.
Energy levels are partly determined by their mass due to the interaction between the electrons and the positively charged nucleus of the atom.
The mass of an electron contributes to its inertia, which is its resistance to changes in motion.
This inertia affects the electron’s ability to respond to the electrostatic attraction from the nucleus and influences its distribution within the atom.
An electron-electron interaction that is mediated by a photon is a process in which two electrons interact with each other through the exchange of a photon. The process is common and is responsible for many of the properties of matter.
Imagine electrons as tiny magnets with an electric field surrounding them. This electric field affects the space around them.
When two electrons are close, their electric fields push against each other because they have the same charge (like poles of magnets repel).
According to the theory of quantum electrodynamics (QED), electrons can’t directly interact with each other. Instead, they exchange energy in the form of a photon, the carrier of the electromagnetic force.
One electron emits a photon due to a fluctuation in its electric field. This photon carries some energy and momentum.
The other electron, influenced by the changing electric field of the first electron (carried by the photon), absorbs the photon. This changes the second electron’s energy and momentum.
So, even though the electrons never directly touch, the exchanged photon acts as a messenger, causing a repulsive force due to the way their electric fields interact.
An electron is a subatomic particle, considered to be an elementary particle, as it doesn’t have any known parts or structure within it.
In an atom, electrons are arranged in orbitals, which are regions in which there is a high probability of them being found. These orbitals are grouped into shells based on their energy levels.
Electrons are the primary carriers of a negative electric charge in an atom, and through their interactions with other particles and fields play an essential role in electromagnetism, electricity, magnetism, chemistry, heat transfer, gravitational and weak interactions.
Electrons significantly contribute to electromagnetism because they have an electric field around them, and when moving produce a magnetic field.
When accelerated electrons radiate or absorb energy in the form of photons.
Electrons can collide with other particles and can be diffracted in a similar way to light.
Electrons exhibit both particle and wave properties. This wave-particle duality is a concept in quantum mechanics and is essential for understanding the behaviour of electrons.
Ectromagnetic energy refers to the energy transported by electromagnetic waves.
The amount of energy carried by an electromagnetic wave is directly proportional to its frequency and inversely proportional to its wavelength.
Radio waves have the longest wavelength and lowest frequency so carry the least energy among electromagnetic waves, while gamma rays have the shortest wavelength and highest frequency in the electromagnetic spectrum and carry the most energy.
Electromagnetic waves are composed of oscillating electric and magnetic fields that travel through space at the speed of light.
Electroluminescence (EL) refers to the phenomenon where light is emitted as a direct result of an applied electric field. Unlike other luminescence mechanisms that rely on chemical reactions or light absorption, electroluminescence is driven solely by the electrical energy itself.
Energy source: An electric field directly interacts with electrons, exciting them to higher energy levels.
Electron transitions: Excited electrons return to their ground state, releasing excess energy as light (determining the colour).
Materials: Certain materials like semiconductors and phosphors are susceptible to electroluminescence under electric fields.
An electrically charged particle is a particle that has a positive or negative charge.
Electrically charged particles are associated with the electromagnetic force, which is one of the four fundamental forces of nature.
The electromagnetic force is responsible for the attraction and repulsion between electrically charged particles, as well as for the propagation of light and other electromagnetic radiation.
All electrically charged particles interact with each other through the electromagnetic force. The strength of the interaction depends on the magnitude of the charges and the distance between the particles.
All particles, except for those with no charge, have a positive or negative charge.
Particles with opposite charges (+ and -) are attracted towards one another by the electric force.
Particles with the same charge (+ and +, or – and -) are repelled away from one another by the electric force.
The charge of a particle is a property that is intrinsic to the particle. It cannot be created, destroyed, or changed.
An electric field line is a component of a diagram representing an electric field. It is a line that corresponds with the electric field at every point along its length.
The direction of the electric field line shows the direction of the electric field.
Drawing field lines to show an electric field is a way of visualizing the direction and strength of the field at given points in space.
Electric field lines point away from their origin at a positively charged object and towards their termination at a negatively charged object.
At a sub-atomic scale, the electric field around a positively charged particle (such as a proton) is directed away from the charge, and the electric field around a negative charge particle (such as an electron) is directed towards the charge.
The density of field lines is used to represent the strength of the electric field, with more lines indicating a stronger field.
An electron orbital is a region of space around the nucleus of an atom where an electron is most likely to be found. Orbitals are not well-defined paths. They represent regions of space where the probability of finding an electron is high.
The arrangement of electrons in shells and orbitals around the nucleus of an atom is governed by the Pauli exclusion principle. The Pauli exclusion principle applies to electrons, protons, neutrons, (and neutrinos). It ensures that no two electrons can occupy the same quantum state simultaneously, leading to the formation of distinct shells and sublevels within each shell, known as orbitals.
The Pauli exclusion principle states that within an atom, each electron occupies a unique quantum state defined by its four quantum numbers: the principal quantum number (n), the azimuthal quantum number (l), the magnetic quantum number (m_l), and the spin quantum number (s).
Four quantum numbers together describe the quantum state of an electron and govern its specific orbital position.
Electron excitation is a general term for any interaction between a photon (particle of light) and an electron. It refers to the process where an electron in an atom or molecule gains energy and jumps to a higher energy level.
Electrons can be excited by various sources, such as:
Photoexcitation: This specifically refers to the absorption of a photon by an electron, leading to the electron’s transition to a higher energy level. This excited state can have various consequences, depending on the system involved. For example, it can trigger chemical reactions, cause fluorescence, or lead to the emission of other photons.
Collisional excitation: This occurs when an energetic particle (another electron, ion, etc.) collides with an electron, transferring some of its kinetic energy to the electron and promoting it to an excited state.
Thermal excitation: When atoms or molecules vibrate due to heat, this energy can be transferred to their electrons, exciting them to higher energy levels. This plays a crucial role in many chemical reactions and physical phenomena.
Chemical excitation: During chemical reactions, the rearrangement of electrons creates and breaks bonds, often resulting in excited states within the participating molecules.
Impact excitation: In certain materials like semiconductors, bombarding the material with high-energy particles (e.g., electrons, ions) can directly excite electrons.
Electrostatic excitation: Applying a strong electric field can create an external force on electrons, potentially pushing them to higher energy levels.
Field ionization: In very strong electric fields, electrons can be ripped out of their atomic or molecular orbitals altogether, resulting in a highly excited state before reaching the vacuum level.
The Bohr model of the atom, proposed by Danish physicist Niels Bohr in 1913, represented a significant development in the understanding of atomic structure. It revolutionized the view in classical physics of the atom by introducing the concept of quantized energy levels for electrons.
While the Bohr model provided valuable insights into atomic structure and spectral lines, it had limitations, especially when applied to larger atoms. It was eventually superseded by quantum mechanics, which provided a more comprehensive understanding of the behaviour of electrons in atoms.
The concept of electron clouds is a part of the quantum mechanical model of the atom, which describes the behaviour of electrons in terms of probability distributions rather than fixed orbits. In this model, electrons are not confined to specific orbits as proposed by the Bohr model but instead are described by electron clouds or probability distributions.
Electrons in an atom are organized into energy levels or shells. These shells are designated by principal quantum numbers (n = 1, 2, 3, …), with higher values of n corresponding to higher energy levels. Electrons in higher energy levels are farther from the nucleus.
Each energy level is divided into subshells or orbitals. These are designated by letters (s, p, d, f). The number of subshells in an energy level is equal to its principal quantum number (n). For example, the first energy level (n = 1) has one subshell (s), the second energy level (n = 2) has two subshells (s and p), and so on.
An orbital is a region in space where there is a high probability of finding an electron. Each orbital can hold a maximum of two electrons with opposite spins. The shape of the orbital depends on its type (s, p, d, f).
The electron cloud is a visual representation of the probability distribution of finding an electron in a particular region around the nucleus. It is not a physical boundary but rather a region where the electron is likely to be found.
The s orbital is spherical and is found in all energy levels. The p orbitals are dumbbell-shaped and are present in the second energy level and higher. The d and f orbitals have more complex shapes and are found in higher energy levels.
The electromagnetic force is one of the four fundamental forces in nature, responsible for various phenomena including electricity, magnetism, and light. It governs the interaction between electrically charged particles, such as electrons and protons. The other forces are the strong nuclear force, the weak nuclear forces and gravity.
The electromagnetic force is a fundamental force, its effects manifest as the push and pull interactions between charged particles.
This means this force is not derived from anything else. It cannot be further broken down or explained by simpler forces.
Even though the nature of the electromagnetic force is not fully understood, classical physics and quantum mechanics can provide a precise understanding of its emergence from charged particles, its behaviour, and the interactions it governs.
It is one of the four most basic and essential forces in the universe. These four forces exist independently of each other, although they can interact and influence each other in certain situations.
Classical electromagnetism is a theory of physics that describes the interaction of electric and magnetic fields at macroscopic scales. It was developed in the late 19th century by physicists such as James Clerk Maxwell and Michael Faraday. Classical electromagnetism precedes quantum physics.
Classical electromagnetism is based on the idea that electric charges and electromagnetic fields are continuous and smooth. It does not take into account the quantization of energy or the wave-particle duality of matter.
Charged particles create electromagnetic fields, which in turn exert electromagnetic forces on other charged particles.
The four Maxwell equations are:
Gauss’s law for electricity: The electric flux through a closed surface is proportional to the total electric charge enclosed by the surface.
Gauss’s law for magnetism: There are no magnetic monopoles, and the magnetic flux through a closed surface is always zero.
Faraday’s law of induction: A changing magnetic field produces an electric field.
Ampere’s circuital law with Maxwell’s correction: A changing electric field or an electric current produces a magnetic field.
These equations can be used to describe a wide range of phenomena, from the propagation of electromagnetic waves to the operation of electrical and electronic devices. They are also used in many different fields, including engineering, medicine, and astronomy.
Electromagnetism is the fundamental force that governs the behaviour of electric and magnetic fields. It encompasses the generation, interaction, and propagation of these fields as electromagnetic waves, and includes the principles and phenomena related to these interactions.
In its broadest sense, electromagnetism refers to the entire realm of phenomena arising from the fundamental electromagnetic force. This includes:
The electromagnetic force itself: The interaction between electrically charged particles, causing attraction or repulsion.
Electromagnetic fields: Invisible fields associated with charged particles and currents, exerting forces on other charged particles.
Electromagnetic radiation: Energy travelling in the form of waves or particles (photons), such as light, radio waves, and X-rays.
In the field of optics, diffusion refers to situations that cause parallel rays of light to spread out more widely. When light undergoes diffusion it becomes less concentrated. Diffuse reflections occur when light scatters off rough or irregular surfaces.
When microscopic features on a surface are significantly larger than the individual wavelengths of light within the visible spectrum, each wavelength of light encounters bumps and ridges exceeding their size.
Instead of reflecting neatly in one direction, the light scatters in different directions.
In this case, scattering doesn’t happen completely randomly. The surface features influence the direction of the scattered light, depending on the angle of incidence and the specific bumps and ridges it encounters.
This scattering creates diffuse reflections, responsible for the soft, uniform illumination seen on textured surfaces like matte paint or unpolished wood.
In the case of a matte phone screen, for example, the light doesn’t form a clear reflection of your face but rather creates a soft, hazy glow due to the diffused light.
A digital screen (or digital display) is an output device for the presentation visual of information. RGB digital screens are used in TVs, computers, phones and projectors.
Digital screens use the RGB (red, green, blue) colour model to represent and display information.
The range of colours that different types of screens can display depends on their technology and specifications.
Many RGB digital screens include light-emitting diodes (LEDs) that can directly or indirectly adjust the intensity of red, green and blue light within each addressable component of the screen to produce pixels of colour that together produce an image.
LEDs are typically used to backlight LCD (liquid crystal display )screens. Different colours are created by colour filters and by adjusting the amount and the polarization of light that is allowed to pass through the crystal sub-pixels that make up each pixel on the screen.
In an OLED display, each pixel provides its own illumination. The organic materials in the OLED emit light when an electric current is applied. Because each pixel can be turned on or off individually, OLED displays can achieve deeper blacks (by completely turning off pixels) and a higher contrast ratio compared to LED-backlit LCD screens.
Digital printing uses the CMYK colour model to enable cyan, magenta, yellow and black inks to be used to output digital files onto paper and other sheet materials.
Digital printers typically overlay highly reflective white paper with cyan, magenta, yellow and black inks or toner.
CMYK is a subtractive colour model suited to working with semi-transparent inks.
Printing has a smaller gamut than TV, computer and phone screens which rely on light emission, rather than reflection of light off sheets of paper.
Digital screens produce comparatively brighter colours than printers because the amplitude of each wavelength of light is larger than can be achieved by a printer.
Digital printers produce dull and less intense colours than digital screens because the amplitude of each wavelength of light is smaller when light is reflected off paper through inks.
Diffuse reflections occur when light scatters off rough or irregular surfaces.
When microscopic features on a surface are significantly larger than the individual wavelengths of light within the visible spectrum, each wavelength of light encounters bumps and ridges exceeding their size.
Instead of reflecting neatly in one direction, the light scatters in different directions.
In this case, scattering doesn’t happen completely randomly. The surface features influence the direction of the scattered light, depending on the angle of incidence and the specific bumps and ridges it encounters.
This scattering creates diffuse reflections, responsible for the soft, uniform illumination seen on textured surfaces like matte paint or unpolished wood.
In the case of a matte phone screen, for example, the light doesn’t form a clear reflection of your face but rather creates a soft, hazy glow due to the scattered light.
In physics, a charged particle is a subatomic particle that possesses an electric charge. This charge can be either positive or negative, and it determines how the particle will interact with other charged particles and with electric and magnetic fields.
Charged particles are the fundamental building blocks of matter. They include electrons, protons, and neutrons, which make up atoms, as well as ions, which are atoms that have lost or gained electrons. Charged particles also include more exotic particles, such as muons and pions, which are found in cosmic rays and in the decay of other particles.
The electric charge of a particle is measured in coulombs (C). An electron has a charge of -1.6×10^-19 C, while a proton has a charge of +1.6×10^-19 C. Neutrons are neutral and have no charge.
Charged particles interact with each others through the electromagnetic force, which is one of the four fundamental forces of nature. The electromagnetic force is responsible for the attraction between oppositely charged particles and the repulsion between like-charged particles. It is also responsible for the behaviour of electric and magnetic fields.
Charged particles are also affected by magnetic fields. A magnetic field exerts a force on a moving charged particle, which can cause the particle to change its direction or speed. This is how electric motors work.
A moving charged particle produces both an electric and a magnetic field. This is because a charged particle will always produce an electric field, but if the particle is also moving, it will produce a magnetic field in addition to its electric field.
The magnetic field is always perpendicular to both the direction in which the charge is moving as well as to the direction of the electric field.
Diffraction of electromagnetic radiation, including visible light, refers to various phenomena that occur when an electromagnetic wave encounters an obstacle or passes through an opening.
Diffraction and interference are phenomena associated with all kinds of waves. Electromagnetic waves are a special case however because of their unique behaviour.
Diffraction of electromagnetic waves deals with the way light bends around the edges of obstacles into regions that would otherwise be in shadow.
Interference deals with the way that electromagnetic waves behave during the diffraction process.
Diffraction can be produced by the edges or by a hole (aperture) in any opaque surface or object.
Diffraction causes a propagating electromagnetic wave to produce a distinctive pattern as waves interfere with one another. The resulting pattern becomes visible if diffracted light subsequently strikes a surface.
Complementary colours are colours that when compared with one another appear to be in complete contrast with one another when viewed by an observer.
Pairs of complementary colours always involve one primary colour and a secondary colour that are opposite one another on a colour wheel. The secondary colour on an RGB colour wheel or HSB colour wheel can always be produced by mixing the other two of the three primaries.
Complementary colours always juxtapose one cool colour with a warm colour. Reds, oranges and yellows are the warm colours, while blues, greens, and purples are the cool colours.
In the context of light, complementary colours result from the additive mixing of wavelengths of light. When all three primary colours are mixed they produce white.
In the context of paints and inks, complementary colours result from the subtractive colour mixing of pigments. When all three primary colours are mixed they produce black.
The mixing of pigments such as powder colours is more complex than mixing known wavelengths of light. When all three primary colours (cyan/magenta/yellow inks or red/yellow/blue powder colours) are mixed they often produce muddy brown or purple colours.
Crown glass is a type of optical glass that does not contain lead or iron. It is used in the manufacture of lenses and other tools and equipment concerned with the visible part of the electromagnetic spectrum.
Crown glass is produced from a mixture of sand, soda ash, and lime.
The sand provides the silica, which is the main component of glass.
The soda ash provides sodium oxide, which lowers the melting point of the glass.
The lime provides calcium oxide, which strengthens the glass.
The potassium oxide is added to give the glass its characteristic optical properties.
Crown glass has a relatively low refractive index, which means that it bends light less than other types of glass. This makes it ideal for lenses that need to transmit a lot of light, such as camera lenses and microscope lenses.
Crown glass also has low dispersion, which means that it bends different colours of light by the same amount. This makes it ideal for lenses that need to produce sharp images, such as telescope lenses and binoculars.
A crest is the highest point of a wave within a wave-cycle. A trough is the opposite of a crest, so it is the lowest point of a wave in a wave-cycle.
On a wave at sea, the crest of a wave is a point where the wave is at its highest. A trough is the opposite of a crest, so a trough is a point where the wave is at its lowest.
In the case of an electromagnetic wave which has an electric and a magnetic axis, a crest on either axis refers to maximum oscillation in the positive direction whilst a trough refers to minimum oscillation in the negative direction.
Wavelength refers to a complete wave-cycle from one crest to the next, or one trough to the next.
Frequency refers to the number of complete waves that pass a given point in a given amount of time.
The amplitude of a wave is a measurement of the distance from the centre line (or the still position) to the top of a crest or to the bottom of a corresponding trough.
Amplitude is related to the energy a wave carries. The energy a wave carries is related to frequency and amplitude. The higher the frequency, the more energy, and the higher the amplitude, the more energy.
The Cosmic Microwave Background (CMB) is a form of electromagnetic radiation dating from an early stage of the universe. is a faint afterglow of the Big Bang, a relic from the very early universe.
The CMB is the oldest known form of radiation and is considered to be evidence for the Big Bang theory of the formation of the Universe.
With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope can detect the CMB as a faint glow that is not associated with any star, galaxy, or other object.
The CMB was initially composed of extremely high-energy gamma rays. However, as the universe expanded and cooled, these gamma rays have been red-shifted, meaning that their wavelengths have been stretched. Today, the CMB appears as microwave radiation.
The CMB is detected as a faint glow of uniform thermal energy coming from all parts of the sky.
The CMB is a relic of the Big Bang, dating back to about 13.8 billion years ago in look-back time.
The phrase look-back time refers to the time it takes for light to travel from its point of origin to our here-and-now.
A continuous spectrum refers to a complete, unbroken range of wavelengths of light.
A continuous spectrum of light is produced by a light source that emits photons over a continuous range of wavelengths.
The spectral colour model deals with a continuous spectrum, it presents colours in a strip, arranged according to their wavelengths, from red at one end to violet at the other.
Sunlight is usually described as a continuous spectrum of colours that make up the visible spectrum with red at one end and violet at the other.
In reality, the spectrum of sunlight is not entirely continuous but has dark lines called absorption lines. These lines correspond with specific wavelengths at which light is absorbed by elements in the Sun’s atmosphere.
The component colours of white light become visible to an observer when the light is dispersed by a prism or a raindrop.
The colours produced by the RGB colour model and the CMY colour model are usually displayed in the form of a colour wheel rather than a strip of colours.
A compound is a substance made from the combination of two or more elements and held together by chemical bonds that are difficult to break. The bonds form as a result of sharing or exchanging electrons between atoms.
A compound (chemical compound) is formed when different elements react, forming bonds between their atoms.
A molecule is the smallest indivisible unit of a compound that retains its chemical properties.
Different elements react and form bonds between their atoms to create a compound.
Compounds have unique properties that are different from the properties of their constituent elements.
Introducing a new element to a compound can lead to additional reactions and the formation of new compounds.
Colour vision is the human ability to distinguish between objects based on the wavelengths of the light they emit, reflect or transmit. The human eye and brain together translate light into colour.
Colour is not a property of electromagnetic radiation, but a feature of visual perception.
The human eye, and so human perception, is tuned to the range of wavelengths of light that make up the visible spectrum and so to the corresponding spectral colours between red and violet.
Light, however, is rarely of a single wavelength, so an observer will usually be exposed to a spread of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
An observer’s perception of colour is a subjective process as the eyes and brain respond together to stimuli produced when incoming light reacts with light-sensitive cells within the retina at the back of the eye.
The perception of colour can be influenced by various factors, such as lighting conditions, surrounding colours, and individual differences in colour perception.
Colour values are the sets of numbers and/or characters used by colour models to systematically identify and store colour information in a form of colour notation recognizable to both computers and humans. Every colour within a colour model is assigned a unique colour value.
The RGB colour model uses both decimal and hexadecimal triplets for colour notation. Each of the three components within a triplet contains a value corresponding to red, green or blue. The colour values within RGB triplets appear as follows:
The colour values in decimal notation for orange: R=255, G=128, B=0.
The colour value in hexadecimal RGB notation for orange: #FF8000.
The HSB colour model uses decimal triplets for colour notation. Each of the three components within a triplet contains a value corresponding to hue, saturation or brightness. The colour values within HSB triplets appear as follows:
The colour values in decimal notation for orange: H=30.12, S=100, B=100.
Colour theory underpins all colour models and all forms of colour management. Some theories explain how human beings perceive colour, others provide practical methods for managing colour in both analogue and digital colour spaces.
Colour theories discussed here at lightcolourvision.org include:
A colour space is a system that defines the gamut of colours available within a specific colour model, the relationship between these colours, and the methods for accurately reproducing them across various devices and workflows.
When combined with devices and software that support colour profiles, colour spaces ensure that colours are accurately reproduced throughout a workflow, from creation to final output. This makes colour spaces essential tools for understanding how different devices will interpret and display digital images.
A colour space defines the range of colours available for an artist, designer, or technician to work with. It can be broad, encompassing a wide spectrum of colours, or narrow, limiting the palette to a specific set. The underlying colour theory and model used in a workflow partially determine the colour space.
Colour spaces are essential for colour management, especially when working across devices in a digital environment. They help ensure consistent colour reproduction across screens and printers. To match a specific device like a projector or printer, you can specify its type and model during the editing process.
When the intended output device is uncertain, adding a colour profile like sRGB or Adobe RGB to a digital file can help guarantee accurate colour reproduction. A colour profile is essentially a set of instructions that tells a device how to interpret and process colour information, ensuring the final output matches the original intent.
In the colour management process, a colour profile is a file containing information that accurately defines a colour space, enabling a device to reproduce the intended range of colours.
Industry-standard colour management uses ICC-compliant colour profiles (or similar). ICC profiles can be recognized by their .icc or .icm file extensions.
Colour profiles address the fact that it may not be possible to reproduce all the colours that an observer sees in an original scene or on-screen when an image is reproduced.
The primary function of a colour profile is to select a colour space that ensures all the colours within an image can be successfully reproduced. In other words, the range of colours output to a device, such as a printer, are adjusted to fit its colour space and ensure they are in-gamut.
Colour profiles can ensure that original colours are managed consistently as an image makes the transition, for example, from a camera through editing to the paper or screen on which it will be displayed.
Editing software such as Adobe Photoshop Lightroom Classic can be set to match the make and model of the camera, the file format, and user-defined settings. These camera-matching profiles ensure that in-camera profiles and picture styles are honoured as they are imported into the editing environment.
The terms brightness and colour brightness have distinct meanings. The first refers to a property of light, and the second to a property of colour as detailed below.
Brightness (as opposed to colour brightness) is used to refer to a property of light.
Colour brightness is used to refer to how much colour something appears to emit or reflect towards an observer.
Colour brightness can be understood as the variation in how a colour is perceived by an observer under well-lit conditions compared to its more muted appearance when in shadow or under poor illumination.
Colour is what humans see in the presence of radiated or reflected light.
The brightness of the colour of an object or surface depends on the intensity of the incident light, as well as the wavelengths of light the object absorbs and reflects.
The colour brightness of a transparent or translucent medium may depend on the intensity of the incident light, the wavelengths of light it absorbs and transmits and the amount it reflects.
Colour brightness can differ depending on the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.
The perception of colour brightness can be influenced by hue, as some hues, such as fully saturated yellow, can appear brighter to human observers than others, like fully saturated red or blue.
Colour constancy is the ability to perceive colours as relatively constant, even under changing lighting conditions.
Colour constancy refers to the perceptual ability to compensate when changes in illumination would otherwise cause things to appear to change colour.
Colour constancy is an extreme case of chromatic adaptation that associates a particular colour with an object regardless of changes in lighting.
Colour vision relies on colour constancy as it allows us to perceive the colour of an object as stable, even when the intensity or spectral distribution of the illumination changes.
Colour constancy contributes to our ability to ignore shifts in an object’s colour when the source or type of light changes, such as when it moves from sunlight to artificial light.
While our visual system is usually successful at maintaining colour constancy, it’s not always perfect, and optical illusions can highlight these imperfections.
A colour management system is a set of techniques and technologies used to ensure that colours are represented and reproduced consistently across different devices (like cameras, monitors, and printers) and in various media.
Colour management systems aim to control the way colours appear from the initial capture or creation, through display and editing, to the final output, ensuring that what you see on your screen or print matches the original colour as closely as possible.
So colour management is about the accurate reproduction of colour.
An artist may want to accurately reproduce a colour they see in a natural scene using oil paints.
A designer may need to identify colours in an original photograph and then ensure they appear the same when printed.
An advertising company must ensure products look the same across all the platforms where consumers encounter them.
A filmmaker may want to use consistent colour grading across every scene within a movie.
A colour model is a system or framework used to understand, organise, and manipulate colour. It ranges from basic concepts, such as the sequence of colours in a rainbow, to more advanced models like RGB, CMYK, and CIE, which are essential for accurate colour reproduction in various fields, including digital media, printing, and manufacturing.
A colour model, underpinned by colour theory, provides a precise and replicable approach to understanding:
How the human eye perceives light and interprets colour.
Different types of colour, including those produced by mixing lights, pigments, or inks.
How to manage the diverse ways colour is processed by devices such as cameras, digital screens, and printers.
Colour models enable us to:
Make sense of colour in relation to human vision and the world around us.
Use colours in logical, predictable, and replicable ways.
Understand how to mix specific colours, whether using lights, pigments, inks, or dyes.
Specify colours using names, codes, notations, or equations.
Organise and apply colour for different purposes, from fabrics and interiors to vehicles.
Colour notation refers to the codes used by colour models to identify and store colour values in a form recognizable to both computers and humans.
Hexadecimal notation is a system for representing RGB colours. For example, a computer display would use the code #FF0000 to produce a bright red pixel. It is commonly used in digital applications such as web design and image processing, allowing for the accurate specification of up to 16,777,216 different colours.
In hexadecimal notation, each of the three RGB colour components—red, green, and blue—is assigned a value between 00 and FF, where 00 represents no intensity and FF represents maximum intensity.
For example:
Red can have a value from 00 to FF (e.g., 00).
Green is also assigned a value between 00 and FF (e.g., 0F).
Blue follows the same pattern (e.g., FF).
HSB colour values (codes) are numeric triplets used in software applications and programming to identify different colours.
A numeric triplet is a code containing three parameters that refer to the hue, saturation, and brightness of a colour.
For example:
The HSB values for pure red are(0, 100%, 100%): Hue: 0°, Saturation: 100%, Brightness: 100%.
A lighter, pastel version of red might be (0, 50%, 100%): Hue: 0°, Saturation: 50%, Brightness: 100%.
A very dark, muted red could be: Hue (0, 100%, 20%): 0°, Saturation: 100%, Brightness: 20%.
The values assigned to the three parameters can be used to define millions of different colours.
Typically, the HSB colour model is implemented as follows:
Hue is represented in degrees from 0 to 360, corresponding to locations on the circumference of a colour wheel.
Saturation is represented as a percentage, where 100% denotes a fully saturated colour, and 0% denotes a fully desaturated colour.
Brightness is represented as a percentage, where 100% denotes the highest luminance of a colour, and 0% denotes the darkest possible shade of a colour.
A chromaticity diagram is a two-dimensional visual depiction of all the colours produced by mixing specific primary colours in a particular colour model.
This means it shows the range of colours achievable by combining red, green, and blue light in varying proportions, not all possible colours imaginable. Some chromaticity diagrams may include colours that are technically visible under specific conditions (e.g., high intensity) but are not typically seen by humans under normal viewing conditions.
The two axes in a chromaticity diagram, typically labelled x and y, represent the proportions of red, green, and blue light needed to produce a specific colour within the model’s gamut.
The most common diagrams, like the CIE 1931 xy diagram, display the range of hues (at varying saturation levels) that a human observer can perceive under ideal conditions.
The scale on each axis of chromaticity diagrams used for technical purposes aligns with the range of colour values (chromaticity coordinates) described by the CIE (1931) XYZ colour space. This enables them to accurately depict colour spaces in a manner consistent with a comprehensive and internationally recognized chromaticity coordinate system.
Some chromaticity diagrams show the smaller range of other colour spaces so that the range of colours that can be reproduced by equipment such as cameras, digital screens and printers can be compared.
Chromaticity diagrams are used to:
Ensure predictable, consistent and accurate colour reproduction across different devices and platforms.
Compare the chromaticity of colours, and so determine the difference between the appearance of particular colours or ranges of colour in terms of hue and saturation.
Assess and optimize the performance of equipment and materials used for colour reproduction.
CMYK is a practical application of the CMY colour model in which black is used alongside the three primary colours (cyan, magenta and yellow) to enable digital printers to produce darker and denser tones.
CMYK refers to the four inks or inked plates used in colour printing: cyan, magenta, yellow, and black. Black is often referred to as the ‘key’ colour because it is used to enhance the depth and detail of the printed image.
The CMYK model works by overlaying colours that partially or entirely mask the background colour which is usually white paper. The inks reduce the amount of light that would otherwise be reflected, thereby creating the desired colours through the absorption of specific wavelengths.
CMY and CMYK are called subtractive colour models because the inks “subtract” the colours red, green and blue from white light. In essence, the inks absorb certain wavelengths of light and reflect others, which combine to produce the perceived colours.
When an observer looks at an image printed using CMYK inks on paper, they see the light that has first passed through the inks to the paper, been reflected off the paper surface, passed through the layers of ink again, and then reached the observer’s eyes. This interaction of light with the ink and paper creates the final visual image.
The CMY colour model deals with a subtractive method of colour mixing. It can be used to explain and provide practical methods of combining three transparent inks and filters (cyan, magenta and yellow) to produce a wide range of other colours and particularly to produce realistic effects when printing digital images onto highly reflective white paper.
The primary colours in the CMY colour model are cyan, magenta and yellow.
The CMY colour model is a subtractive colour model used with transparent or translucent inks or filters.
Meanwhile, the CMYK colour model (sometimes called four-colour or process printing) uses the same three primary colours as CMY but uses a fourth component, black ink (K), to increase the density of darker colours and blacks.
The CMYK colour model along with its system of notation enables an exact and reproducible approach to colour printing and other similar applications.
The CMYK colour model is deeply embedded in all contemporary digital printer technologies and underpins industrial standards for the printing industry.
The CIE 1931 XYZ colour space (also known as CIE 1931 colour space) was one of the first mathematically defined colour spaces and was adopted by the International Commission on Illumination (CIE) as its standard method.
The CIE XYZ colour space was the first comprehensive method for systematizing the relationship between colour stimuli and human colour perception.
In an experimental situation, the CIE XYZ colour space can match any colour an observer sees with a known mixture of wavelengths of light.
The foundation of the CIE XYZ colour space is the ability to identify the precise mixture of wavelengths of light needed to stimulate cone cells to produce any colour experience within the visible spectrum.
Viewed diagrammatically the CIE XYZ colour space takes the form of a graph showing a volume of colour corresponding with every wavelength in the visible spectrum. The location of every colour is determined in relation to the x and y axes of the graph. The two axes are used to identify the coordinates for each colour within this two-dimensional vector space.
The coordinates themselves are derived from tristimulus colour values.
With the development of the CIE XYZ colour space, trichromatic colour models and their corresponding colour spaces provide methods for anticipating and managing colour reproduction in every applicable field and type of technology.
In terms of colour management, the trichromatic colour theory underpins device-independent additive colour spaces such as the sRGB colour space and the Adobe RGB colour space and device-dependent additive colour models such as RGB, HSB and CMYK and their corresponding colour spaces.
Classical physics (or classical mechanics) is a group of physics theories that predate modern, more complete, and more widely applicable theories associated with quantum physics (quantum mechanics).
Classical physics describes many aspects of nature at an everyday scale but neglects to explain things at very small (sub-atomic) and very large (cosmological) scales. It is a very successful theory, and many of its predictions have been experimentally verified.
Classical physics studies the motion of macroscopic objects, from projectiles to parts of machinery and astronomical objects such as spacecraft to the movement of planets and stars.
For objects governed by classical physics, if the present state is known, it is possible to predict how it will move in the future (determinism), and how it has moved in the past (reversibility).
Classical physics has its roots in:
Newtonian mechanics – Isaac Newton, 17th century
Thermodynamics – Carnot, Joule and Kelvin, 19th century
Chromaticity refers to the characteristic of colour when described in terms of hue and saturation, rather than just its wavelength.
Chromaticity refers to the quality of a colour that sets it apart from white, grey, or black.
The chromaticity of different colours is often described by chromaticity coordinates that define where a colour appears within a colour space.
The simplest way to understand chromaticity is through a chromaticity diagram that creates a two-dimensional visual display of all the colours produced by a specific colour space.
A chromaticity diagram displays hue and saturation without mentioning their brightness.
The most common chromaticity diagrams showcase the full range of colours visible to a human observer under ideal conditions. The position of each colour is plotted using the range of colour values (chromaticity coordinates) described by the CIE (1931) XYZ colour space.
Some chromaticity diagrams illustrating the CIE (1931) XYZ colour space include overlays of the smaller gamuts of colour spaces associated with different mediums, lighting conditions, and devices.
Examples of colour spaces with smaller gamuts than the CIE (1931) XYZ colour space include:
Chromatic dispersion is the process where light, under specific conditions, splits into its constituent wavelengths, and the colours linked with each wavelength become visible to a human observer.
Chromatic dispersion is the result of the connection between wavelength and refractive index..
When light moves from one medium (like air) to another (like water or glass), each wavelength is influenced to a varying extent based on the refractive index of the involved media. The outcome is that every wavelength changes its direction and speed.
If the light source emits white light, the individual wavelengths spread out, with red at one end and violet at the other.
A familiar example of chromatic dispersion is when white light strikes raindrops and a rainbow becomes visible to an observer.
A chemical bond is a durable attraction between atoms, ions or molecules that enables the formation of chemical compounds.
A chemical bond may result from:
The electric force between negatively and positively charged ions as seen in ionic bonds.
Via the sharing of electrons, as is the case with covalent bonds.
The material world is bound together by chemical bonds, which determine the structure, size and characteristics of chemical compounds.
A chemical compound consists of two or more atoms from different elements that are chemically bonded together.
Chemical bonds occur because the electromagnetic force operates between charged particles.
Opposite charges attract one another and like charges repel.
The higher the charge, the stronger the force.
There are different types of chemical bonds. Each affects the physical and chemical properties of a compound, including reactivity, melting point, boiling point, and electrical conductivity.
In general terms, a centreline is a real or imaginary line that passes through the centre of something, often dividing the object into two halves.
In a wave diagram used to illustrate electromagnetic waves, a centreline may be used to show either:
Point of intersection: This is the ideal centerline and represents the point where the electric and magnetic fields cross zero simultaneously. This point stays constant as the wave propagates.
Halfway between crest and trough: This is a common but simpler representation used for ease of visualization. It doesn’t always coincide with the point of field intersection in certain wave types or when considering polarization.
The terms brightness and colour brightness have distinct meanings. The first refers to a property of light, and the second to a property of colour as detailed below.
Brightness (as opposed to colour brightness) is used to refer to a property of light.
Colour brightness is used to refer to how much colour something appears to emit or reflect towards an observer.
Colour brightness can be understood as the variation in how a colour is perceived by an observer under well-lit conditions compared to its more muted appearance when in shadow or under poor illumination.
In the HSB colour model:
Hue refers to the perceived difference between colours and is usually described using names such as red, yellow, green, or blue.
Saturation refers to the vividness of a colour compared to an unsaturated colour.
Brightness refers to the perceived difference in the appearance of colours under ideal sunlit conditions compared to poor lighting conditions where a hue’s vitality is lost.
Brightness can be measured as a percentage from 100% to 0%.
As the brightness of a fully saturated hue decreases, it appears progressively darker and achromatic.
The terms brightness and colour brightness have distinct meanings. The first refers to a property of light, and the second to a property of colour as detailed below.
Brightness (as opposed to colour brightness) is used to refer to a property of light.
Colour brightness is used to refer to how much colour something appears to emit or reflect towards an observer.
When brightness is used in connection with the HSB colour model it is used alongside hue and saturation and refers to the method of selecting and adjusting colours in software applications such as Adobe Illustrator.
The HSB colour model is a representation of colours that combines hue, saturation, and brightness components.
In the HSB brightness represents the intensity or lightness of a colour, with higher values indicating brighter or lighter colours.
An object that absorbs all radiation falling on it, at all wavelengths, is called a blackbody.
A blackbody is a theoretical concept for an object that completely absorbs all electromagnetic radiation, regardless of factors such as angle of incidence, wavelength, frequency, or amplitude.
A perfect blackbody doesn’t exist in reality. However, certain objects and materials, such as stars and carbon in soot or graphite behave almost like blackbodies.
When a blackbody emits electromagnetic radiation, the spectral distribution of the emissions is dependent solely on its temperature.
The radiation emitted by a black body is known as blackbody radiation.
If enough heat is applied to a blackbody, it will begin to appear orange at a certain point, and as the temperature increases, it changes from white to pale blue and then to light blue.
Bipolar cells are the retinal interneurons that provide the primary pathway from photoreceptors (rod and cone cells) to ganglion cells.
In addition to directly transmitting signals from photoreceptors to ganglion cells, they connect to amacrine cells that assist in integrating information and forming a comprehensive picture of an entire visual scene.
There are approximately a dozen types of bipolar cells, all of which serve as centres for integration.
Each type of bipolar cell acts as a dedicated channel for information about light, collected by either a single or a small group of rod or cone cells.
Each type of bipolar cell interprets and relays its own version of information gathered from photoreceptors to ganglion cells
Auroras are caused by the interaction between charged particles (such as electrons), ejected from the Sun (solar wind), with the Earth’s magnetosphere.
The magnetosphere accelerates electrons as they enter the atmosphere after travelling from the Sun
The colour and pattern of an aurora are partly determined by the degree of acceleration given to the particles as they enter the atmosphere.
Different gases in Earth’s atmosphere produce different colours of auroras when struck by solar particles. Oxygen produces green and red light, while nitrogen gives blue and purple.
The shape of an aurora depends on the Earth’s magnetic field lines, as the charged particles travel along these lines.
The visibility of auroras depends not only on geographical location and time of day but also on solar activity. Stronger solar winds typically cause more intense auroras.
An atom is the smallest unit of a chemical element that retains all of its physical and chemical properties.
At the core of an atom is a nucleus that contains protons which are positively charged sub-atomic particles. The number of protons defines the atomic number and thus the chemical element of the atom. For instance, a hydrogen atom has one proton.
In addition to protons, the nucleus of an atom also houses neutrons, sub-atomic particles with a mass slightly larger than protons but with no electrical charge.
Circling the nucleus are negatively charged particles called electrons, which are kept in place by their attraction to the positively charged protons in the nucleus.
When discussing rainbows, angular distance is the angle between the line from the observer to the centre of the rainbow (rainbow axis) and the line from the observer to a specific colour within the arc of a rainbow.
Angular distance is one of the angles measured on a ray-tracing diagram that illustrates the sun, an observer, and a rainbow from a side view.
Think of angular distance as the angle between the line to the centre of a rainbow down which an observer looks and the line to a specific colour in its arc. The red light is deviated by about 42.4° and violet light by about 40.7°.
An artificial light source is any source of light created by humans, as opposed to natural light sources like the sun or stars. Artificial light sources are generated by converting different forms of energy into light.
There are several major categories of artificial light sources such as:
Incandescent: These work by heating a filament until it glows, emitting light (traditional light bulbs).
Fluorescent: Electric current triggers gas inside the bulb to produce ultraviolet light, which a phosphor coating converts into visible light.
LED (Light-Emitting Diode): Electricity excites semiconductors, causing them to emit light.
Gas-discharge lamps: Electric current passes through a gas, producing bright light (e.g., neon signs, street lamps).
When discussing the formation of rainbows, the angle of deflection measures the angle between the initial path of a light ray before it hits a raindrop, and the angle of deviation, which measures how much the ray bends back on itself in the course of refraction and reflection towards an observer.
The angle of deflection and the angle of deviation are always directly related to one another and together add up to 180 degrees.
The angle of deflection equals 180 degrees minus the angle of deviation. So, it’s clear the angle of deviation is always equal to 180 degrees minus the angle of deflection.
In any particular case, the angle of deflection is always the same as the viewing angle because the incident rays of light that form a rainbow all follow paths that run parallel with the rainbow axis.
Alexander’s band, also known as Alexander’s dark band, is an optical phenomenon observed in rainbows. It refers to the region between the primary and secondary bows, which often appears noticeably darker to an observer compared to the rest of the sky.
The areas of the sky around a rainbow may appear blue or grey depending on weather conditions and cloud cover. However, these areas outside, inside, and between the primary and secondary rainbows tend to have distinct tonal differences from one another:
The area inside the arcs of a primary rainbow always appears tonally lighter than the surrounding sky.
The area outside primary and secondary rainbows appears darker.
The area between primary and secondary rainbows appears the darkest – this is Alexander’s band.
Alexander’s band can be explained by the fact that fewer photons are directed from this specific area of the sky toward an observer.
Airglow is a faint, continual emission of light originating from the Earth’s upper atmosphere, typically between 80 and 400 kilometres in altitude. While often mistaken for distant starlight, it forms a distinct phenomenon with unique characteristics and scientific significance.
Emission Process: Airglow primarily results from chemiluminescence. Solar radiation ionizes atmospheric molecules like oxygen, nitrogen, and sodium during the day. These excited molecules later recombine with other particles at night, releasing energy as light in specific wavelengths.
Spectral Colours: Different molecules emit light at characteristic colours:
Green: Primary emission from excited oxygen atoms.
Red: Mainly from sodium atoms, contributing to the reddish band above the horizon.
Blue and violet: Emissions from hydroxyl (OH) and nitric oxide (NO) molecules.
Visibility and Variations: Airglow intensity varies due to altitude, wavelength, and location. Magnetic storms can enhance brightness, creating spectacular displays. Astronauts observe airglow as a luminous band encircling Earth.
The general purpose of a colour space is to determine the range of colours available within a specific workflow and may be determined by a user or programmatically.
The Adobe RGB (1998) colour space aims to ensure the optimal range of colours available within the RGB colour model are accurately reproduced when output to digital displays or printers.
In a digital environment, the aim is to ensure that a selected range of colours appears consistent throughout a workflow and that the desired range of colours is successfully reproduced at the end of the process.
Achromatic means without colour so refers to surfaces or objects that appear white, grey or black. Achromatic colours lack hue or saturation but can be described in terms of their brightness.
When mixing paint, achromatic colours are produced by adding black and/or white until the original colour is fully desaturated.
Achromatic colours are produced on digital screens by mixing red, green and blue light in equal proportions.
Accommodation refers to the way the lenses inside our eyes accommodate for the fact that objects of interest may be close to or at a distance. Sharp images on the retina are the result of modifying the focal length of each lens.
The focal length of a lens is the distance at which it brings parallel rays of light into focus on the retina.
A lens with a shorter focal length brings objects closer to the eye into focus and has a wider field of view.
A lens with a longer focal length brings objects at a greater distance from the eye into focus and has a narrower field of view.
Absorption of light occurs when the frequency of the wave matches the frequency of electrons orbiting atomic nuclei in a material.
When light is absorbed by an object or medium, its energy excites electrons, causing them to vibrate more vigorously and collide with other atoms, which in turn produces heat.
Materials selectively absorb photons whose frequencies match their frequencies.
Reflected light bounces off a surface at the same wavelength with little or no change in energy.
Additive colour is shorthand for the additive mixing of wavelengths of light to produce colour. The method involves mixing wavelengths corresponding with primary colours at varying intensities and projecting them onto a surface or screen. When seen by an observer, light enters and stimulates the eyes and, depending on the intensity of the signal on each channel, produces the visual impression of a predicted colour.
Whilst additive colour mixing is the method used to combine wavelengths of light, subtractive colour mixing is the method used with dyes, inks and pigments.
An additive approach to colour mixing is used in the case of the emission of light by light-emitting diodes (or similar light sources) embedded into the screens of mobile phones, computers and televisions etc.
An additive approach to colour mixing is also used with digital projectors. In this case, sufficient light must be produced on each channel to form intense images when focused onto a screen across a room.
RGB colour is one of the additive colour models that combine wavelengths of light corresponding with red, green and blue primary colours to produce other colours.
Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce other colours. Red green and blue are often described as being components of the resulting colour.
Different colours are produced by varying the intensity of the component colours between fully off and fully on.
When fully saturated red, green and blue primary colours are combined, they produce white.
When any two fully saturated additive primaries are combined, they produce a secondary colour: yellow, cyan and magenta.
Some RGB colour models can produce over 16 million colours by fine-tuning the intensity of each of the three primary colours.
The additive RGB colour model cannot be used for mixing different colours of pigments, paints, inks, dyes or powders. To combine these colourants subtractive colour models are used.
Amacrine cells interact with bipolar cells and/or ganglion cells. They are a type of interneuron that monitor and augment the stream of data through bipolar cells and also control and refine the response of ganglion cells and their subtypes.
Amacrine cells are in a central but inaccessible region of the retinal circuitry. Most are without tale-like axons. Whilst they clearly have multiple connections to other neurons around them, their precise inputs and outputs are difficult to trace. They are driven by and send feedback to the bipolar cells but also synapse on ganglion cells, and with each other.
Amacrine cells are known to serve narrowly task-specific visual functions including:
Efficient transmission of high-fidelity visual information with a good signal-to-noise ratio.
Maintaining the circadian rhythm, so keeping our lives tuned to the cycles of day and night and helping to govern our lives throughout the year.
Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism) which enables edge detection and contrast enhancement.
Object motion detection which provides an ability to distinguish between the true motion of an object across the field of view and the motion of our eyes.
Centre-surround antagonism refers to the way retinal neurons organize their receptive fields. The centre component is primed to measure the sum-total of signals received from a small number of cones directly connected to a bipolar cell. The surround component is primed to measure the sum of signals received from a much larger number of cones around the centre point. The two signals are then compared to find the degree to which they agree or disagree.
Bipolar cells, a type of neuron found in the retina of the human eye connect with other types of nerve cells via synapses. They act, directly or indirectly, as conduits through which to transmit signals from photoreceptors (rods and cones) to ganglion cells.
There are around 12 types of bipolar cells and each one functions as an integrating centre for a different parsing of information extracted from the photoreceptors. So, each type transmits a different analysis and interpretation of the information it has gathered.
The output of bipolar cells onto ganglion cells includes both the direct response of the bipolar cell to signals derived from photo-transduction but also responses to those signals received indirectly from information provided by nearby amacrine cells that are also wired into the circuitry.
We might imagine one type of bipolar cell connecting directly from a cone to a ganglion cell that simply compares signals based on differences in wavelength. The ganglion cell might then use the information to determine whether a certain point is a scene is red or green.
Not all bipolar cells synapse directly with a single ganglion cell. Some channel information that is sampled by different sets of ganglion cells. Others terminate elsewhere within the complex lattices of interconnections within the retina so enabling them to carry packets of information to an array of different locations and cell types.
The distance between the retina (the detector) and the cornea (the refractor) is fixed in the human eyeball. The eye must be able to alter the focal length of the lens in order to accurately focus images of both nearby and far away objects on the retinal surface. This is achieved by small muscles that alter the shape of the lens. The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance remains constant.
The ability of the eye to adjust its focal length is known as accommodation. The eye accommodates by assuming a lens shape that has a shorter focal length for nearby objects in which case the ciliary muscles squeeze the lens into a more convex shape. For distant objects, the ciliary muscles relax, and the lens adopts a flatter form with a longer focal length.
Colour is something we see every moment of our lives if we are conscious and exposed to light. Some people have limited colour vision and so rely more heavily on other senses – touch, hearing, taste and smell.
Colour is always there whether we are aware and pay attention to it or not. Colour is what human beings experience in the presence of light. It is important to be clear about this. Unless light strikes something, whether it is air, a substance like water, a physical object or the retina at the back of our eyes, light, as it travels through space, is invisible and so has no colour whatsoever. colour is an artefact of human vision, something that only exists for living things like ourselves. Seeing is a sensation that allows us to be aware of light and takes the form of colour.
The experience of colour is unmediated. This means that it is simply what we see and how the world appears. In normal circumstances, we feel little or nothing of what is going on as light enters our eyes. We have no awareness whatsoever of the chemical processes going on within photosensitive neurons or of electrical signals beginning their journey towards the brain. We know nothing of what goes on within our visual cortex when we register a yellow ball or a red house. The reality is, we rarely even notice when the world disappears as we blink! In terms of immediate present perception, colour is simply something that is here and now, it is that aspect of the world we see as life unfolds before us and is augmented by our other senses, as well as by words, thoughts and feelings etc.
It takes about 0.15 seconds from the moment light enters the human eye to conscious recognition of basic objects. What happens during this time is related to the visual pathway that can be traced from the inner surface of the eyeball to the brain and then into our conscious experience. The route is formed from cellular tissue including chains of neurons some of which are photosensitive, with others tuned to fulfil related functions.
So, let’s start at the beginning!
Before light enters the eye and stimulates the visual system of a human observer it is often reflected off the surfaces of objects within our field of view. When this happens, unless the surface is mirror-like, it scatters in all directions and only a small proportion travels directly towards our eyes. Some of the scattered light may illuminate the body or face of an observer or miss them completely. Some is reflected back off the iris enabling others to see the colour of their eyes. Sometimes light is also reflected off the inside of our eyeballs – think of red-eye in flash photography.
Cross-section of the human eyeball
The fraction of light that really counts passes straight through the pupil and lens and strikes the retina at the back of the eyeball. From the point of view of an observer, this leads to two experiences:
Things an observer sees right in the centre of their field of vision, which is to say, whatever they are looking at.
Things an observer sees in their peripheral vision and so fill the remainder of a scene.
If we think of light in terms of rays, then the centre of the field of vision is formed from rays that enter our eyes perpendicular to the curvature of the cornea, pass right through the centre of the pupil and lens and then continue in a straight line through the vitreous humour until they strike the retina. Because these rays are perfectly aligned with our eyeballs they do not bend as they pass through the lens and so form an axis around which everything else is arranged. The point where this axis strikes the retina is called the macula and at its centre is the fovea centralis where the resulting image appears at its sharpest.
Peripheral vision is formed from rays that are not directly aligned with the central axis of the cornea and pupil, and do not pass through the very centre of the lens. All the rays of light around this central axis of vision change direction slightly because of refraction.
It deserves mention at this point that the lenses in each eye focus in unison to accommodate the fact those things we scrutinise most carefully may be anywhere from right in front of our noses to distant horizons.
We must also not forget that the optical properties of our lenses mean that the image that forms on the retina is both upside down and the wrong way round.
On a sunny day, if you stand with the Sun at your back and look at the ground, the shadow of your head will align with the antisolar point.
The antisolar point is the position directly opposite the Sun, around which the arcs of a rainbow appear. An imaginary straight line can always be drawn that passes through the Sun, the eyes of an observer, and the antisolar point, which is the geometric centre of a rainbow.
This concept corresponds with what an observer sees in real life: the idea that a rainbow has a center. From a side view, the centre of a rainbow is called the antisolar point, so named because it is opposite the Sun relative to the observer’s position.
Unless observed from the air, the antisolar point is always below the horizon. Both primary and secondary rainbows share the same antisolar point, as do higher-order bows, such as fifth and sixth-order rainbows.
A colour wheel is a circular diagram divided into segments, featuring primary colours, and used to visualize the result of colour mixing.
Colour wheels can enhance understanding of colour relationships and assist with the accurate selection and reproduction of colours.
A colour wheel starts with segments representing primary colours. Additional segments are added between them to explore the outcome of mixing adjacent primary colours.
By adding more segments between existing ones, further mixing of adjacent colours can be explored.
A colour wheel exploring the additive RGB colour model starts with red, green, and blue primary colours.
A colour wheel exploring the subtractive CMY colour model starts with cyan, magenta, and yellow primary colours.
The critical angle for light approaching the boundary between two different media is the angle of incidence above which it undergoes total internal reflection. The critical angle is measured with respect to the normal at the boundary between two media.
Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
Internal reflection takes place when light travelling through a medium:
Strikes the boundary with another medium that has a lower refractive index
At an angle greater than the critical angle
For example, internal reflection takes place when light reaches air from glass and at an angle greater than the critical angle.
In normal conditions, light is partially refracted and partially reflected because of irregularities in the surface at the boundary.
It is only when the angle of incidence is greater than the critical angle for all points along a boundary that total internal reflection takes place.
In the field of optics, dispersion is shorthand for chromatic dispersion which refers to the way that light, under certain conditions, separates into its component wavelengths, enabling the colours corresponding with each wavelength to become visible to a human observer.
Chromatic dispersion refers to the dispersion of light according to its wavelength or colour.
Chromatic dispersion is the result of the relationship between wavelength and refractive index.
When light travels from one medium (such as air) to another (such as glass or water) each wavelength is refracted differently, causing the separation of white light into its constituent colours.
When light undergoes refraction each wavelength changes direction by a different amount. In the case of white light, the separate wavelengths fan out into distinct bands of colour with red on one side and violet on the other.
Familiar examples of chromatic dispersion are when white light strikes a prism or raindrops and a rainbow of colours becomes visible to an observer.
Energy is a property of matter and fields, which can be transferred between systems or transformed into different forms but cannot be created or destroyed.
Everything contains energy including all forms of matter and so all objects.
Energy is evident in all forms of movement, interactions between, and changes to the forms and properties of matter.
At an atomic level, energy is evident in the movement of electrons around the nucleus of an atom. Energy is stored in the nucleus of atoms as a result of the forces that bind protons and neutrons together.
Energy can be transferred between objects, and converted from one form to another, but cannot be created or destroyed.
Everything in the universe uses energy in one form or another.
When it comes down to it, matter is energy.
Light has energy but no mass so does not occupy space and has no volume.
Energy is often described as either being potential energy or kinetic energy.
An electronvolt (eV) is a unit of energy commonly used in atomic, nuclear, and particle physics to measure the energy carried by individual particles and electromagnetic radiation. It’s a convenient unit because the energies involved in these fields are much smaller than those we encounter in everyday life.
Electronvolts can be used to measure the energy of elementary particles, including photons, which are the smallest units of electromagnetic radiation (quanta of the electromagnetic field).
One electronvolt is equal to the energy gained by a single electron when it is accelerated across a potential difference of 1 volt.
Photons (quanta of light) travelling through this same potential difference would also gain 1 eV of energy.
The electronvolt is not part of the SI unit system. The SI unit for energy is the joule (J). However, joules are too large for many particle-level interactions.
1 electronvolt (eV) is equivalent to approximately 1.602 x 10^-19 joules (J).
Electric fields are a property of photons. These dynamic fields, along with corresponding magnetic fields, are responsible for the transmission of electromagnetic energy, such as visible light.
Photons are massless particles that carry electromagnetic energy, with each photon representing a quantum of light. The electric fields produced by photons oscillate, meaning their strength varies cyclically between maximum and minimum values over time.
The frequency of the electric field determines the frequency of the photon. A higher photon frequency corresponds to a shorter wavelength, as frequency and wavelength are inversely related.
A material gets its colour as molecules absorb some wavelengths of light and reflect others. The colour an observer sees corresponds with the reflected wavelengths.
The part of a molecule that determines the colour an observer sees is called the chromophore.
The colour produced by a surface or object corresponds with wavelengths of light that are not absorbed during their interaction with the chromophore.
Electric and magnetic fields are two interrelated aspects of the electromagnetic force, a fundamental force of nature. This force governs the attraction and repulsion between electrically charged particles, and its fields are responsible for the propagation of light and other forms of electromagnetic radiation.
The connection between force and field can be summarized as:
A force pushes or pulls an object. A field is a region of space in which a force acts on objects.
In the case of the electromagnetic force, the electric field is the region of space in which electric forces act on charged particles, and the magnetic field is the region of space in which magnetic forces act on moving charged particles.
In other words, electric and magnetic fields are the regions in space where the electromagnetic force pushes and pulls microscopic objects such as electrons and macroscopic objects such as magnets or metal filings.
An electromagnetic wave carries electromagnetic radiation.
An electromagnetic wave is formed as electromagnetic radiation propagates from a light source, travels through space and encounters different materials.
Electromagnetic waves can be imagined as synchronised oscillations of electric and magnetic fields that propagate at the speed of light in a vacuum.
Electromagnetic waves are similar to other types of waves in so far as they can be measured in terms of wavelength, frequency and amplitude.
We can feel electromagnetic waves release their energy when sunlight warms our skin.
Remember that electromagnetic radiation can be described either as an oscillating wave or as a stream of particles, called photons, which also travel in a wave-like pattern.
The notion of waves is often used to describe phenomena such as refraction or reflection whilst the particle analogy is used when dealing with phenomena such as diffraction and interference.
Electromagnetic radiation is a type of energy more commonly simply called light. Detached from its source, it is transported by electromagnetic waves (or their quanta, photons) and propagates through space at the speed of light.
Electromagnetic radiation (EM radiation or EMR) includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
Man-made technologies that produce electromagnetic radiation include radio and TV transmitters, radar, MRI scanners, microwave ovens, computer screens, mobile phones, all types of lights and lamps, electric blankets, electric bar heaters, lasers and x-ray machines.
At the quantum scale of electromagnetism, electromagnetic radiation is described in terms of photons rather than waves. Photons are elementary particles responsible for all electromagnetic phenomena.
The term quantum refers to the smallest quantity into which something can be divided. A quantum of a thing is indivisible into smaller units so they have no sub-structure. A photon is a quantum of electromagnetic radiation.
A single photon with a wavelength corresponding with gamma rays might carry 100,000 times the energy of a single photon of visible light.
The electromagnetic spectrum includes electromagnetic waves with all possible wavelengths of electromagnetic radiation, ranging from low-energy radio waves through visible light to high-energy gamma rays.
There are no precisely defined boundaries between the bands of electromagnetic radiation in the electromagnetic spectrum.
The electromagnetic spectrum includes, in order of increasing frequency and decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Visible light is only a very small part of the electromagnetic spectrum.
The perception of colour by an observer results from properties of light that are visible to the human eye. The visual experience of colour is associated with terms like red, blue and yellow.
The observation of colour depends on:
The range and intensity of wavelengths of visible light emitted by a light source, and the various media and materials it encounters on its journey to the retina of a human eye
Optical phenomena such as absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.
Predispositions of an observer, such as their personal and social experience, health and state of mind.
Cone cells, or cones, are a type of neuron (nerve cell) in the retina of the human eye.
Cone cells are cone-shaped whilst rod cells are rod-shaped.
Cone cells are responsible for colour vision and function best in bright light, as opposed to rod cells, which work better in dim light.
Cone cells are most concentrated towards the macula and densely packed in the fovea centralis, but reduce in number towards the periphery of the retina.
There are believed to be around six million cone cells in the human retina.
An observer perceives bands of colour when visible light separates into its component wavelengths and the human eye distinguishes between different colours.
The human eye and brain together translate light into colour.
When sunlight is dispersed by rain and forms a rainbow, an observer will typically distinguish red, orange, yellow, green, blue and violet bands of colour.
Although a rainbow contains electromagnetic waves with all possible wavelengths between red and violet, some ranges of wavelengths appear more intense to a human observer than others.
The amplitude of an electromagnetic wave is directly connected with the amount of energy it carries.
In a wave diagram, the amplitude is represented as the distance from the centre line (or midpoint) of a wave to the top of a crest or the bottom of a corresponding trough.
When the amplitude of an electromagnetic wave increases, the overall distance between any peak and the next trough also increases.
The quantity of energy carried by an electromagnetic wave is proportional to the amplitude squared.
Amplitude has an indirect correlation with the perception of the intensity of light and the brightness of colour as perceived by an observer because additional factors such as phase and interference must be taken into account.
An additive colour model explains how different coloured lights (such as LEDs or beams of light) are mixed to produce other colours.
Additive colour refers to the methods used and effects produced by combining or mixing different wavelengths of light.
The RGB colour model and HSB colour model are examples of additive colour models.
Additive colour models such as the RGB colour model and HSB colour model can produce vast ranges of colours by combining red, green, and blue lights in varying proportions.
An additive approach to colour is used to achieve precise control over the appearance of colours on digital screens of TVs, computers, and phones.
An electromagnetic field is a physical field that describes the behaviour of electrically charged particles and their interactions. It is a region of space where electric and magnetic forces are present. These fields are created by the presence or movement of electrically charged particles.
An electromagnetic field results from the coupling of an electric and magnetic field.
When an electromagnetic field experiences a change in voltage or current its reconfiguration into an electromagnetic wave can be described in terms of wavelength, frequency and energy.
An electromagnetic wave can be thought to come into existence when a static electric field experiences a change in voltage or a static magnetic field experiences a change in current producing radiating oscillations of electromagnetic energy that propagate through space.
The difference between an electromagnetic field and an electromagnetic wave is that the wave has a non-zero frequency component which is the source of the energy it transports.
Electromagnetic radiation is essentially the result of an oscillating electromagnetic field propagating through space.
Cross-section of the human eyeball