Particle physics

Particle physics is a branch of physics that studies the fundamental constituents of matter and energy and their interactions.

  • Particle physics is an experimental subfield of quantum mechanics often associated with the Large Hadron Collider at CERN.
  • Large Hadron Collider is a particle accelerator, which collides particles at high energies. These collisions create new particles, which are then studied by detectors. The detectors measure the properties of the new particles, such as their mass, energy, and charge.
  • The Standard Model serves as the theoretical framework employed by particle physicists to investigate fundamental forces and particles.
  • Among the most important discoveries of particle physics was the discovery of the photon and the dual nature of light (both a wave and particle).


Particles & diagram conventions

About particles & diagram conventions
  • Absorption: When a photon is absorbed by an atom or molecule, it can be depicted in a diagram by showing the photon arrow disappearing and a dashed line indicating the location of the absorbing particle.
  • Coherence: Coherence is a property of light waves that determines how well they maintain a constant phase relationship with each other over time and distance. This can be shown in a diagram by adding multiple photon arrows with the same wavelength and direction, and a dashed line connecting them to indicate their coherence.
  • Diffraction: When light passes through a small opening or aperture, it can exhibit diffraction, causing the light to spread out and form a pattern of interference. This can be shown with a diagram that includes a narrow slit or aperture, with the photon arrows bending and spreading out as they pass through it.
  • Directional arrows: The direction of travel of the photon is indicated by a directional arrow. The arrow usually points in the direction of the photon’s motion.
  • Dispersion: When representing the dispersion of light, the diagram may show a beam of white light passing through a prism and separating into its component colours. This can be depicted by a series of arrows of different lengths and colours diverging from a central point.
  • Emission: Conversely, when a photon is emitted by an atom or molecule, it can be shown by adding a photon arrow and a wavy line to represent the emitted radiation.
  • Energy: The energy of the photon is often represented by the size or thickness of the arrow. Larger or thicker arrows represent photons with more energy, while smaller or thinner arrows represent photons with less energy.
  • Frequency: The frequency of the photon is indicated by the number of arrows present in a given space or time. More arrows indicate higher frequency, while fewer arrows indicate lower frequency.
  • Interactions: Diagrams may also show the interaction of photons with matter, such as when a photon is absorbed or emitted by an atom. This interaction is often represented by a curved arrow pointing towards or away from the atom.
  • Interference: When two or more photons interact and interfere with each other, it can be shown by adding multiple photon arrows with different colours or lengths, and lines connecting them to represent constructive or destructive interference.
  • Polarization: The polarization of a photon can be indicated by a double-headed arrow, with one arrow pointing up and the other pointing down to represent vertically polarized light. Alternatively, horizontal polarization can be indicated with a double-headed arrow pointing to the left and right.
  • Polarization filters: Polarization filters are materials that allow only certain orientations of light waves to pass through them while blocking others. This can be depicted in a diagram by adding a filter with horizontal or vertical lines to represent the preferred orientation of the filter.
  • Phase: The phase of a light wave refers to its position relative to a reference wave at a given point in time and space. This can be represented in a diagram by showing a photon arrow with a wavy line representing the wave, and a dotted line indicating the phase at a particular point.
  • Photon: The light particle is represented by a symbol called a “photon.” A photon is often depicted as a small circle or dot.
  • Quantum states: In quantum mechanics, photons can exist in multiple states simultaneously, a property known as superposition. This can be shown in a diagram by adding multiple photon arrows with different energies, wavelengths, or directions, and lines connecting them to represent their superposition.
  • Reflection: When a photon reflects off a surface, it can be depicted by showing the photon arrow bouncing off the surface at an angle equal to the angle of incidence.
  • Refraction: When a photon passes through a transparent material, such as glass or water, it can be depicted in a diagram by showing the photon arrow bending as it enters the material at an angle other than 90 degrees.
  • Scattering: Scattering of light can be shown by a photon arrow changing direction after interacting with a particle or molecule. This change in direction is often represented by a curved arrow that deviates from the straight path of the original photon.
  • Wavelength: The wavelength of the photon is indicated by the length of the arrow representing the photon. Longer arrows indicate photons with longer wavelengths, while shorter arrows indicate photons with shorter wavelengths.

Perceived colour

Perceived colour refers to what an observer sees in any given situation and so is a subjective experience.

  • Our ability to perceive and distinguish between colours is crucial to how we experience and understand the world.
  • Perceived colour is influenced by the range and mixture of wavelengths and intensities of light that enter the eye.
  • Perceived colour can be influenced by the properties of light, objects, and the attributes of visual perception.
  • Colour perception tends to prioritize information that is important to an observer, but it may not always be objectively accurate.
  • The perceived colour of an object can be influenced by factors, such as the size, shape, and structure of objects, their position and orientation, and the direction of incident light.
  • Colour perception can be described by chromatic colour names (such as pink, orange, brown, green, blue, and purple) and achromatic colour names (such as black, grey or white).
  • Perceived colour is often a combination of chromatic and achromatic content.
  • The state of adaptation of an observer’s visual system can affect colour perception, such as when fatigued cells in the retina produce after-images.
  • An observer’s expectations, priorities, current activities, recollections, and previous experiences can all influence perceived colour.
  • The International Commission on Illumination (CIE) defines perceived colour as having three main characteristics: hue, brightness (or lightness), and colourfulness (or saturation/chroma).
  • Here at we often characterise colour in terms of hue, saturation and brightness and so align our discussions with the HSB colour model.


Photoluminescence is the emission of light from a material after it absorbs light. This process involves exciting the electrons in the material to a higher energy level, followed by their relaxation back to a lower energy level, releasing energy in the form of light.

Photoluminescence in Three Steps
  1. Absorption: Light excites electrons in a material, pushing them to higher energy levels.
  2. Relaxation: Excited electrons gradually lose energy.
  3. Light Emission: Lost energy is emitted as light of a different wavelength (usually longer).
Key Points
  • Emitted light often has lower energy (longer wavelength) than absorbed light due to energy loss.
  • Different types of photoluminescence exist based on relaxation mechanisms:
    • Fluorescence: Quick relaxation, emitting light soon after absorption.
    • Phosphorescence: Slower relaxation, emitting light later, creating a “glow” effect.
    • Chemiluminescence: Light emission triggered by a chemical reaction initiated by light absorption.
  • Fluorescent lamps convert ultraviolet light to visible light.
  • LEDs use electroluminescence (photoluminescence triggered by electricity).
  • Biological markers help visualize structures or processes in cells.
  • Quantum dots have bright, tunable photoluminescence for displays and solar cells.
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Photometry is the science concerned with measuring the human perception of light.

  • Measuring human visual responses to light is not straightforward because the eye is a complex and intricate organ.
  • An internationally recognized system of measurements, known as the CIE system, was established in 1931 by the Commission Internationale de l’Eclairage (CIE).
  • The Commission established the typical spectral responsiveness of the human eye to wavelengths across the visible spectrum and compiled the data into a photopic curve.
  • The CIE’s photopic curve shows that, in bright light, the strongest response of the human eye is to the colour green with less sensitivity towards the spectral extremes, red and violet.
  • A second set of measurements of the typical responsiveness of the human eye to wavelengths across the visible spectrum at low levels of light, (where determining colour differences is difficult), resulted in data compiled into the scotopic curve.
  • Having defined the spectral response of the human eye, the CIE sought a standard light source to measure luminous intensity.
  • Luminous intensity is a measure of how bright a light source appears to the human eye, and it is typically used to describe the brightness of light sources such as light bulbs, lamps, and LEDs.
  • The first source, which led to the development of the terms footcandle and candlepower, was a specific type of candle commonly used in the 18th and 19th centuries before standardised artificial light sources were developed.
  • In 1948, the standard was redefined for better repeatability. It was named the International Candle and defined as the amount of light emitted from a given quantity of melting platinum.


A photon is the basic building block of light. A photon is a single indivisible bundle (particle or wave) of energy within an electromagnetic field.

  • In the field of optics, light is explained in terms of waves (wavelength, frequency and energy) but this description doesn’t always fit the evidence. It became clear during the 20th century that light sometimes exhibits wave-like behaviour, at others both waves and particles, or just particles.
  • Contemporary physics considers that electromagnetic fields propagate through space configured as bundles of energy. These are bundles of photons.
  • Photons are the force carriers of radiant energy (electromagnetic radiation).
  • A photon is a type of elementary particle and represents a quantum of light (eg. visible light). Another way of putting this is that a photon is the smallest quantity (quantum, plural quanta) into which light can be divided.


A photon is a particle that carries electromagnetic radiation. It is the fundamental unit of light.

  • Thinking of photons as particles is useful for understanding the quantum nature of light.
  • In the world of quantum physics, photons are the fundamental constituents of all forms of electromagnetic radiation, including light. They serve as the carriers of the electromagnetic force.
  • Photons are elementary particles that have no mass and no electric charge. They are the quanta of the electromagnetic field, which is the fundamental field that describes electromagnetic interactions. Electromagnetic radiation, including light, is a manifestation of the electromagnetic field.

  • Photons are the carriers of electromagnetic force because they are the only particles that can mediate electromagnetic interactions. When two charged particles interact electromagnetically, they exchange photons. The exchange of photons gives rise to the electromagnetic force.

  • Photons have no rest mass and always travel at the speed of light in a vacuum.

  • Photons exhibit both wave-like and particle-like properties, a characteristic referred to as wave-particle duality. This duality is inherent to quantum particles, causing light to behave as a wave under certain conditions, as both waves and photons in others, and strictly as particles in yet others.
Photons and mass
  •  The statement above about zero rest mass can be broken down as follows:
    • According to the theory of relativity, any object that has mass needs energy to accelerate.
    • The amount of energy required to accelerate an object increases as the object’s mass increases.
    • Photons are unique in that they have zero rest mass and this means they do not require any energy to be accelerated.
    • As a result, they always move at the speed of light in a vacuum.
    • So photons always travel at approximately 299,792 kilometres per second and, when undisturbed, they never decelerate or come to a halt.
Energy of photons, wavelength, frequency and colour
  • The energy of a photon (its photon energy) is intrinsically linked to its wavelength and frequency, and in perceptual terms, to its colour.
    • Photon Energy: The energy of a photon, E, is given by the equation E=hf, where h is Planck’s constant and f is the frequency of the photon. So, a photon with a higher frequency has higher energy.
    • Frequency and Wavelength: Frequency (f) and wavelength (λ) are related by the speed of light (c), through the equation c=fλ. So, photons with a higher frequency have a shorter wavelength, and photons with a lower frequency have a longer wavelength.
    • Colour Perception: In terms of colour perception, different energies (and thus frequencies and wavelengths) of photons are perceived as different colours. For example, photons with high energy (high frequency, short wavelength) are perceived as blue/violet, while photons with lower energy (low frequency, long wavelength) are perceived as red. The range of frequencies (or wavelengths) that human eyes can detect is known as the visible light spectrum.
Photons and interaction with charged particles
  • Photons can interact with charged particles such as electrons within atoms. In such events, they can either be absorbed, resulting in the elevation of the particle to a higher energy state, or be emitted when a particle transitions from a higher energy state to a lower one.
  • A higher energy state refers to a quantum state or level of an atomic or subatomic system in which an electron or particle has absorbed energy and moved to a more excited or elevated position, typically farther from the nucleus or centre of the system.
Photons and interaction with matter
  • Photons can engage with matter through various processes. They can be scattered, absorbed, or emitted during interactions with atoms and molecules. These processes are crucial for a range of phenomena from the heating of surfaces under sunlight to the transmission of information in fibre optic cables.
Photons and polarization
  • Photons can be polarized. This means their electric and magnetic fields can oscillate in a specific orientation. Polarization is used in various applications, from LCD screens to polarized sunglasses, and is also a significant aspect of certain quantum mechanical phenomena.
Photons and their Energy, Frequency, Wavelength and Momentum
  • The energy of a photon determines its frequency, wavelength and momentum. This energy can be transferred during interactions, leading to phenomena like fluorescence and the photoelectric effect. In the visible spectrum, different energies (and therefore frequencies and wavelengths) correspond to different colours of light.
  • The energy of a photon is directly proportional to its frequency. This means that photons with higher frequencies have more energy. The energy of a photon is also inversely proportional to its wavelength. This means that photons with longer wavelengths have less energy. The momentum of a photon is inversely proportional to its wavelength. This means that photons with shorter wavelengths have more momentum.
Photons and wave-particle duality
  • Wave-particle duality refers to the phenomenon where entities like light can exhibit characteristics of both waves and particles.
  • Electromagnetic radiation, including light, is often described using wave properties. However, when it interacts with matter, it behaves like particles.
  • A photon is a quantum of electromagnetic radiation and represents the smallest discrete amount of light energy.
  • When a photon is absorbed by matter, the energy becomes localized at specific points. This phenomenon is termed ‘wave function collapse.’ It describes the transition of a quantum system from a superposition of states to a definite state upon measurement.
  • Wave-particle duality is a fundamental aspect of quantum mechanics and applies to all particles, not just light. Particles like electrons also exhibit wave-like and particle-like behaviour.
  • The double-slit experiment is an experiment in quantum physics that demonstrates the wave-like behaviour of particles, including photons and electrons, and is a key illustration of wave-particle duality.

Photon energy

About the energy in photons
  • According to the equation: E = hf, the amount of energy photons possess is directly proportional to their frequency (f) and inversely proportional to their wavelength (λ) – where E is the energy of the photon, h is Planck’s constant, and f is the frequency of the photon.
  • Therefore, photons with higher frequencies (and shorter wavelengths) have more energy than photons with lower frequencies (and longer wavelengths).
  • The energy of a photon is quantized, meaning it can only take on certain discrete values based on the relationship between its frequency and Planck’s constant.

Photon energy

Photon energy is the energy carried by a single photon. The energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength.

  • The higher the photon’s frequency, the higher its energy. Equivalently, the shorter the photon’s wavelength, the higher its energy.
  • Photon energy is determined solely by the photon’s frequency and wavelength.
  • Other factors, such as the intensity of the radiation, do not affect the energy of individual photons. In other words, two photons of light with the same frequency have the same energy, regardless of their source. even if one was emitted from a wax candle and the other from the Sun.
  • The electronvolt (eV) and the joule are the units commonly used to express the energy of photons.
  • The energy of a photon can also be expressed:
    • In terms of its wavelength or frequency using Planck’s constant (h). E = hν = hc/λ, where E is the energy, ν is the frequency, λ is the wavelength, and c is the speed of light.
    • As a quantum of electromagnetic radiation.

Photon properties

About the properties of photons
  • A photon is a type of elementary particle that is a quantum (plural = quanta) of the electromagnetic field. This means that it is the smallest quantity into which light can be divided.
  • A photon carries energy and can be described both in terms of a particle and a wave.
  • While the wave model of light works well for some phenomena, the particle model is necessary to explain others.
  • Light can exhibit both wave-like and particle-like behaviour depending on the experiment performed. This is known as wave-particle duality.
  • The wavelength of a photon determines its energy and frequency.
  • Photons with longer wavelengths have lower energy and frequency, while photons with shorter wavelengths have higher energy and frequency.
  • The wavelength of a photon can also affect its behaviour, such as its ability to penetrate materials or cause photochemical reactions.

Other properties of photons include:

  • Photons have zero rest mass but have energy and momentum proportional to their frequency.
  • Unlike other kinds of elementary particles, photons have no rest mass.
  • Photons are electrically neutral, meaning they have no electric charge.
  • Photons are stable particles that do not decay over time.
  • Photons can interact with other particles, such as electrons, through processes such as absorption and emission.
  • Photons can interact with other particles, such as electrons, through processes like absorption and emission.
  • Photons always travel at the speed of light in a vacuum, regardless of their frequency or energy.

Photon-electron interaction

In photon-electron interactions, a photon can either be absorbed by an electron or scattered by it. During the interaction, the photon transfers some or all of its energy and momentum to the electron.

  • One of the most common interactions used to explain the link between electromagnetism and visible light is the photon-electron interaction.
  • The specific outcome of a photon-electron interaction depends on the photon’s energy and the electron’s state. For example, if the photon has enough energy, it can knock the electron out of its orbit. This is known as the photoelectric effect. If the photon does not have enough energy to knock the electron out of its orbit, it can be scattered by the electron. This is known as Compton scattering.
    • On absorption of a photon by an electron, the electron gains energy and transitions to a higher energy level – a higher orbit around the nucleus of an atom.
    • When an electron scatters a photon, the photon alters its trajectory and might lose some energy. However, the electron’s energy level remains unaffected.
  • A photon can transfer all of its energy to an electron, even if the photon has more energy than the electron’s binding energy. In this case, the electron will be ejected from the atom with excess kinetic energy.
  • The amount of energy a photon can transfer to an electron is capped by its energy. A photon cannot impart more energy to an electron than it possesses.
  • The likelihood of a photon being absorbed by an electron hinges on both the photon’s energy and the electron’s energy level. A higher photon energy increases the absorption likelihood, while a higher electron energy level decreases the absorption probability. (The likelihood of absorption also depends on the polarization of the photon and the orientation of the electron’s orbital.)
  • The likelihood of a photon being scattered by an electron is similarly influenced by photon energy and electron energy level.  (The likelihood of scattering also depends on the angle of photon-electron collision and the spin of the electron). \
  • Examples of a photons-electron interaction include:
    • Photoelectric Effect: When a photon with adequate energy impacts an electron, the electron can be expelled from the atom. Called the photoelectric effect, the photon’s energy must exceed the electron’s binding energy within the atom.
    • Compton Scattering: In a collision between a photon and an electron, the photon can scatter. The photon might relinquish energy during the collision, and the electron can acquire a portion of the photon’s energy.
    • Chromophore excitation: The photon-chromophore interaction accounts for the observable colour of objects.
      • The interaction between photons and chromophores is more intricate than photon-electron interaction. It encompasses energy redistribution among all electrons in a molecule and can result in various outcomes such as fluorescence, phosphorescence, and energy transfer.
      • The interaction between photons and chromophores is generally referred to as molecular excitation or chromophore excitation, distinct from photon-electron interaction but can still be considered a type of photon-electron interaction since it involves the interaction of photons with the electrons in chromophore molecules.
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Photons & electrons

About photons & electrons
  • Photons are fundamental particles and are considered to be the primary constituent of visible light and all other types of electromagnetic radiation.
  • Photons are considered to be pure energy, they have no mass or electric charge and are dimensionless.
  • Since photons exist at the subatomic level alongside other fundamental particles such as electrons, quarks, leptons and gauge bosons, they exist outside the range of our everyday perception.
  • However, when photons interact with electrons, the effects can be seen by the human eye.
  • For example, when light travels through air or water, it interacts with the electrons of the atoms and molecules it encounters, causing different wavelengths to scatter in various directions. This scattering of light is visible to the human eye when we see the blue colour of the sky or the red colour of a sunset.
  • If electrons had eyes, they would likely perceive photons as disturbances in the electric and magnetic fields that pervade the space around them.
  • In the event of a collision between a photon and an electron, the interaction might lead the electron to absorb the energy of the photon and transition to a higher energy state or to emit a new photon as it returns to a lower energy state.
  • These types of interactions are frequently described as “excitation” or “de-excitation” processes and can happen without any significant release of energy or the production of any visible effects.

Contemporary quantum mechanics brings another perspective to the way a photon interacts with an electron.

  • Photons are the carriers of electromagnetic force.
  • When a photon interacts with an electron, the interaction can take many different forms, depending on the energy and nature of the photon, and the specific properties of the electron and the surrounding environment.
  • In some cases, the interaction can cause the electron to take in the energy of the photon and move to a higher energy level, or to give off a new photon as it returns to a lower energy level.
  • The interaction between a photon and an electron must be described in terms of the probabilistic nature of quantum mechanics because the precise outcome of the interaction cannot be predicted with certainty. Only the probabilities of different outcomes can be calculated.
  • Other useful concepts that quantum mechanics brings to this interaction are:
  • Particle-Wave Duality: As both particles and waves, photons and electrons can be described using wavefunctions that give the probability of finding the particle at any point in space. The interaction isn’t deterministic, where you can exactly predict the outcome, but probabilistic, meaning you can calculate the likelihood of different outcomes.
  • Energy Levels and Quantum Jumps: The absorption of a photon’s energy by an electron is probabilistic. The electron has a certain probability to absorb the energy and “jump” to a higher level. If the electron is in a superposition of states (a fundamental concept in quantum mechanics), it may or may not absorb the photon depending on how the wavefunction collapses upon measurement.
  • Wavefunction: This provides the probabilities of the outcomes of measurements of a quantum system. It essentially describes everything that can be known about a quantum system. The act of measuring these properties causes the wavefunction to collapse to a particular state.
  • Wavefunction collapse: When a measurement is made on a quantum system, the wavefunction “collapses” into a state that is consistent with the outcome of the measurement. This state is one of the possible states that the system could have been in before the measurement, according to the superposition principle.
  • Superposition: According to quantum mechanics, particles can be in multiple positions and states simultaneously until a measurement is made, a property known as superposition.
    • Until a measurement is made, a particle doesn’t have a definite position but rather exists in a spread of possible positions.
    • Similar to positions, a quantum particle can exist in a superposition of other states. For example, an electron in an atom can exist in a superposition of spin states (spin up and spin down), or a superposition of energy states.
  • Uncertainty Principle: Because of the inherent uncertainty in position and momentum, the exact outcome of an interaction between a photon and an electron can’t be predicted with absolute certainty. We can only talk about probabilities.
  • Probability and Wavefunction Collapse: The interaction between the photon and the electron could result in various outcomes, each with its own probability, defined by the wavefunction. The exact outcome isn’t known until the wavefunction collapses upon measurement. This is inherently a probabilistic event.
  • Quantum Entanglement: If a photon and electron become entangled, the state of one particle will instantly affect the state of the other, no matter how far apart they are. This correlation is perfectly deterministic, but the outcome of measurements on each individual system is probabilistic, giving rise to the paradoxical nature of entanglement.

Photons, electric & magnetic fields

About photons, electric and magnetic fields
Photons & electric fields
  • Photons and electric fields are intimately connected in the framework of electromagnetic radiation.
  • When an electric field oscillates or changes, it generates electromagnetic waves.
  • This oscillation of the electric field gives rise to the emission of photons.
  • The energy of each photon is directly proportional to the frequency of the electric field oscillation.
  • Higher-frequency oscillations produce photons with higher energy, while lower-frequency oscillations produce photons with lower energy.
  • When photons interact with charged particles or materials with electric fields, they can be absorbed, transmitted, or scattered.
  • This interaction leads to phenomena such as the photoelectric effect, where photons can eject electrons from a material by transferring their energy to the electrons, and optical phenomena like reflection and refraction.
Photons & magnetic fields
  • When a photon interacts with a magnetic field, it can cause the magnetic field to oscillate, creating an electromagnetic wave.
  • This is because a magnetic field is one component of the electromagnetic field, which also includes an electric field.
  • When a photon interacts with the magnetic field, it transfers energy to it, causing the magnetic field to oscillate back and forth.
  • The oscillation of the magnetic field, in turn, creates an oscillating electric field, and the two fields together form an electromagnetic wave that propagates through space at the speed of light.
  • Fields refer to regions in space where a physical quantity is present that can exert a force or influence on other objects or particles.
  • Fields are used in physics to describe how certain properties or forces can vary with position and time.
  • In the case of electromagnetic fields, they represent the distribution of electric and magnetic properties in space.
  • An electric field is associated with electric charges and describes the force experienced by other charges (positive or negative) in its presence. Electric fields exist in the vicinity of charged objects and can exert forces on other charged particles.
  • A magnetic field is associated with moving electric charges, such as electric currents in wires, and it describes the force experienced by other moving charges (currents) in its vicinity. Magnetic fields exist around current-carrying conductors and can interact with moving charges, causing them to experience a magnetic force.
  • Electric, and magnetic forces, can be represented using vector fields.
    • In the case of an electric vector field, the vectors represent the electric force that a charged object would experience at different points in space due to the presence of other electric charges.
    • In the case of a magnetic vector field, the vectors represent the magnetic force that a moving charged object would experience at different points in space due to the presence of magnetic fields.
  • Fields are often represented in two dimensions using field lines.
    • The density of field lines indicates the strength of the field at a particular point – the more dense the lines, the stronger the field.
  • The conventions for how to show gravitational, electric, and magnetic field lines are all slightly different to model the unique aspects of each force. Some common models are shown below.

Photopic curve

A photopic curve is a graphical representation of the sensitivity of the human eye to light under normal, bright lighting conditions. It indicates that the human eye has the strongest response to green light, with less sensitivity to the red and violet ends of the visible spectrum.

  • The standard photopic curve used in the CIE 1931 colour space is based on the photopic luminosity function, which describes the average sensitivity of the human eye to different wavelengths of light under normal, bright lighting conditions.
  • A photopic luminosity function is a mathematical function used to derive the photopic curve from the CIE 1931 colour space.
  • The CIE 1931 colour space is a standardized system for describing colours based on human colour perception. It was developed by the International Commission on Illumination (CIE) in 1931 and is still widely used today.
  • In low light conditions, the sensitivity of the human eye to light changes, and the scotopic curve is used to describe the response of the eye to light.
  • Scotopic and photopic curves have different units of measurement.
    • A photopic curve uses units of luminous flux, which is a measure of the total amount of visible light emitted by a source.
    • A scotopic curve, on the other hand, uses units of luminous intensity, which is a measure of the brightness of a light source per unit of solid angle.

Pigment epithelium

Pigment epithelium is a layer of cells at the boundary between the retina and the choroid of the human eye that nourish neurons with the retina.

  • Pigment epithelium is firmly attached to the underlying choroid on one side but less firmly connected to retinal visual cells on the other.
  • The choroid is the layer of connective tissue that supports the retina.

Pigment epithelium

Pigment epithelium

Pigment epithelium is a layer of cells at the boundary between the retina and the eyeball that nourish neurons within the retina. It is firmly attached to the underlying choroid is the connective tissue that forms the eyeball on one side but less firmly connected to retinal visual cells on the other.


A pixel is the smallest addressable element in a digital image that can be uniquely processed and is defined by its spatial coordinates and colour values.

  • A pixel, also known as a picture element, is a physical point in a digital image and the smallest addressable element of a display device.
  • In the editing process, a pixel is the smallest controllable element of a digital image.
  • Many digital displays, including LCD screens, contain LEDs arranged in a grid pattern and emit light when an electrical current is passed through them, allowing them to display different colours and brightness levels.
  • OLED displays use a different technology that uses organic compounds that emit light when an electrical current is passed through them.
  • The RGB colour model is commonly used for still images displayed on digital screens, such as computer monitors and televisions.
  • In the RGB colour model, each pixel is composed of three subpixels that control the red, green, and blue colour channels.
  • By varying the light emitted by an LED, every pixel can display a wide range of colours and shades, allowing for the creation of highly detailed and vibrant images on screen.
  • The resolution of a digital screen, or the number of pixels it can display, is an important factor in determining its overall image quality and sharpness.
  • Higher-resolution screens can display more pixels per inch (PPI), resulting in smoother, more detailed images with less visible pixelation.
  • Newer display technologies may use variations of the RGB colour model to display still images, such as RGBW (Red, Green, Blue, White) or RGBY (Red, Green, Blue, Yellow).

Plank constant

The Planck constant is a fundamental constant of nature that is denoted by the symbol h.

  • The Planck constant is a measure of the smallest possible amount of energy that can be carried by a single quantum of electromagnetic radiation (a photon).
  • The Planck constant is also related to the wavelength of a photon by the equation E = hf, where E is the energy of the photon, f is its frequency, and h is the Planck constant.
  • The equation, energy (E) = Planck constant (h) x frequency (f), allows the quantity of energy associated with electromagnetic radiation to be calculated if the frequency is known.
  • The Planck constant is used extensively in modern physics, particularly in the fields of quantum mechanics, atomic physics, and condensed matter physics.
  • It plays a crucial role in determining the energy levels of atoms and molecules, as well as the behaviour of subatomic particles such as electrons and photons.
  • The value of the Planck constant is approximately 6.626 x 10^-34 joule-seconds (Js).

Polarization & rainbows

About plane polarization & rainbows
  • Plane polarization is one of the optical effects that account for the appearance of rainbows.
  • Plane polarization, also known as linear polarization, is a characteristic of electromagnetic waves, including light.
  • The polarization of light in rainbows contributes to the vividness and intensity of the colours we see.
  • The light that produces rainbow effects is typically 96% polarized. This means that:
    • Observed light exiting a raindrop is polarized on a plane bisecting each droplet and tangential to the arcs of the rainbow.
  • The presence of other atmospheric phenomena, such as water droplets of varying sizes or ice crystals, can affect the amount of plane polarization and so influence the appearance of rainbows.


Polychromatic refers to something that contains or displays multiple colours.

  • In the context of light, polychromatic refers to light that contains multiple wavelengths or colours
  • White light which is a combination of all colours in the visible spectrum is polychromatic.
  • The vibrant colours of a sunset or the many shades of green in a forest are polychromatic.
  • The opposite of polychromatic is monochromatic, which refers to something that is composed of only one colour or hue.