Dictionary tag: C

Classical electromagnetism

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.

Charged particle

  • 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.

Complementary colour

  • 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

  • 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.

Crest

  • 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.

Cosmic Microwave Background

  • 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.

Continuous spectrum

  • 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.

Compound

  • 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

  • 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 value

  • 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

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:
    • CMY colour model
    • Greyscale colour model
    • HSB colour model
    • Lab colour model
    • RGB colour model
    • Spectral colour model
    • Trichromatic colour model

Colour space

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.

 

Colour profile

  • 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.

Colour brightness

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

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.

Colour management system

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.

Colour model

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

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.

Chromophore

A chromophore is the part of a molecule that produces its colour.

  • Things appear to have colour because they absorb certain wavelengths of light while reflecting others.
  • When wavelengths of light within the visible spectrum enter the human eye, the observer perceives this as colour.
  • The chromophore is the part of a molecule where there is an energy difference between two different molecular orbitals.
  • A molecular orbital refers to the position and wave-like behaviour of an electron as it moves around an atom’s nucleus.
  • If the energy difference of a chromophore falls within the range of the visible spectrum (2 to 2.75 electron volts) then it will produce colour.
  • The colour produced by a surface or object corresponds with wavelengths of light that are not absorbed during their interaction with the chromophore.

Chromaticity diagram

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 colour model

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.
  • Find out more here https://lightcolourvision.org/dictionary/definition/cmyk-colour-model/

CMY colour model

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.
  • Find out more here https://lightcolourvision.org/dictionary/definition/cmy-colour-model/

CIE 1931 XYZ colour space

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

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
    • Maxwell’s electromagnetism, 19th century.

Chromaticity

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:
    • Adobe RGB (1998)
    • Prophoto RGB
    • sRGB
    • 2200 matt paper

Chromatic dispersion

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.

Chemical bond

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.

Centreline

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.

Charge

Electric charge is a fundamental property of matter that governs its interaction with electric and magnetic fields.

  • Electric charge carriers, protons (+) and electrons (-) are the primary charge carriers in matter.
  • There are two types of electric charge:
    • Positive charge: Carried by protons, found in the nucleus of atoms.
    • Negative charge: Carried by electrons, which exist in orbitals around the nucleus.
  • Neutons, the other particles within the nucleus of an atom, have no charge.

Colour wheel

  • 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.

Critical angle

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.

Colour of objects

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.

Colour

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 cell

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.