Dictionary category: Definitions

Bands of colour

An observer perceives bands of colour when visible light separates into its component wavelengths and the human eye distinguishes between different colours.

In the presence of a rainbow, an observer will typically see six bands of colour (red, orange, yellow, green, blue and violet) rather than a unique colour corresponding with each wavelength.

  • When sunlight is dispersed by rain and forms a rainbow, an observer will typically distinguishes 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 phenomenon of perceiving distinct colour bands is typically attributed to the characteristics of human colour vision, or as an artefact of human colour vision.
  • There is no property belonging to the visible part of the electromagnetic spectrum that that results in the appearance of bands of colour to an observer.
  • The visible spectrum is composed of a continuous range of wavelengths between red and violet that produce a continuous range of corresponding colours.
  • In experimental situations, human observers can distinguish between spectral colours corresponding with many hundreds of different wavelengths of light.
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Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

https://en.wikipedia.org/wiki/Rainbow

Bioelectroluminescence

Bioelectroluminescence is a type of chemiluminescence where the emission of light is specifically produced by a biological process within a living organism.

  • Bioelectroluminescence is a type of chemiluminescence where the emission of light is specifically produced by a biological process within a living organism.
  • It is found in many marine organisms (fireflies, jellyfish, anglerfish), as well as some terrestrial species like fungi.
  • It involves specialized biochemical reactions within the organism. Typically, a molecule called luciferin reacts with oxygen in the presence of an enzyme called luciferase.
  • The reaction releases energy in the form of visible light, creating the bioluminescent glow.
Light sources
Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Adhesive tape peeling
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms		Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars		Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles		Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests		Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs		ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks		Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters		Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights		ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
Pressure-sensitive adhesives		TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests		PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • Bioelectroluminescence is a type of chemiluminescence where the emission of light is specifically produced by a biological process within a living organism.
  • It is found in many marine organisms (fireflies, jellyfish, anglerfish), as well as some terrestrial species like fungi.
  • It involves specialized biochemical reactions within the organism. Typically, a molecule called luciferin reacts with oxygen in the presence of an enzyme called luciferase.
  • The reaction releases energy in the form of visible light, creating the bioluminescent glow.

Bioluminescence

Bioluminescence is a type of luminescence resulting from the production and emission of electromagnetic radiation by living organisms. Bioluminescence, meaning “living light,” occurs in a wide variety of creatures, from bacteria and fungi to fish, and insects. Unlike artificial light sources, bioluminescence doesn’t involve heat generation but involves a chemical process within the organisms themselves.

  • The chemistry behind bioluminescence lies in a special molecule called luciferin. When luciferin reacts with another molecule, usually an enzyme called luciferase, energy is released in the presence of oxygen. This energy takes the form of visible light, creating the characteristic glow. Different luciferins and luciferases determine the emitted light’s colour and intensity.
  • Marine organisms, including single-cell dinoflagellates, some jellyfish, and deep-sea anglerfish, use the luciferin-luciferase reaction to release energy as blue-green light.
  • Terrestrial Organisms, including fireflies and glowworms, also use the luciferin-luciferase reaction to create light.
    • Fireflies have a specialised light organ that contains luciferin and luciferase. The reaction involves adenosine triphosphate to provide the energy to excite an electron in the luciferin, which then emits a photon upon returning to its ground state.

    • Glowworm larvae of various fly and beetle species possess luciferin and luciferase and produce a similar light-emitting reaction.

Materials Involved in Bioluminescence
  • Luciferin: This small organic molecule acts as the fuel, with different variations depending on the organism. It’s the source of the chemical energy that gets converted into light.
  • Luciferase: This enzyme acts as the catalyst, speeding up the reaction between luciferin and oxygen. Different luciferases determine the colour and intensity of the emitted light.
  • Oxygen: Although not universally used, oxygen serves as the final electron acceptor in most bioluminescent reactions, stabilising the energy released and enabling light emission.
  • Adenosine Triphosphate (ATP): In some systems, like fireflies, ATP provides the initial energy boost to excite the electron in luciferin, kicking off the bioluminescent reaction. However, in these systems, the excited electron ultimately interacts with oxygen as the final electron acceptor, similar to systems that use oxygen directly.
The Bioluminescent Process
  • Excitation: The luciferase enzyme interacts with luciferin, either directly using energy from ATP (fireflies) or indirectly drawing energy from sunlight or chemical reactions within the organism (dinoflagellates). This interaction excites an electron in the luciferin molecule, raising its energy state.
  • Electron Transfer to oxygen: In most bioluminescent reactions, the excited electron in the luciferin molecule doesn’t directly emit light. Instead, it transfers its energy through a series of steps, ultimately reaching oxygen as the final electron acceptor.
  • Energy Release: During this transfer, some of the released energy is channelled into a specific form suitable for light emission. This specific energy then triggers the luciferin molecule to emit a photon (light).
  • Light Emission: Upon releasing this converted energy as a photon (light), the excited molecule returns to its ground state.
  • Variations: While the basic principle remains similar, specific molecules, reaction pathways, and light colours can vary significantly depending on the organism and its unique bioluminescent system.
Electron Excitation in Bioluminescence
  • Imagine an electron within the luciferin molecule orbiting its nucleus, like a ball at the bottom of a hill. This is the ground state, where the electron has its lowest energy.
  • During the bioluminescent reaction, the luciferin molecule receives energy from different sources depending on the organism:
    • Fireflies: The luciferase enzyme uses ATP to directly excite the electron, pushing it to a higher energy level.
    • Dinoflagellates: Light or chemical reactions interact with luciferin, indirectly causing electron excitation.
    • Once energised, the excited electron jumps to a higher energy level within the luciferin molecule. This excited state is unstable, and the electron wants to return to its ground state.
    • However, it doesn’t simply drop back down. Instead, in most bioluminescent organisms, it transfers its energy through a series of steps to oxygen, the final electron acceptor. The excited electron doesn’t physically jump to oxygen but interacts with it indirectly through the molecular chain, ultimately transferring its energy.
    • During this transfer, some of the released energy gets converted into a specific form suitable for light emission. Finally, this converted energy triggers the emission of a photon from the excited molecule (often luciferin itself), releasing the remaining energy and bringing the electron back to its ground state.
Light sources
Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Adhesive tape peeling
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms		Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars		Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles		Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests		Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs		ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks		Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters		Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights		ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
Pressure-sensitive adhesives		TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests		PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • Bioluminescence, meaning “living light,” is the production and emission of light by living organisms. It occurs in a wide variety of creatures, from bacteria and fungi to fish, insects, and even some deep-sea animals. Unlike artificial light sources, bioluminescence doesn’t involve heat generation, making it a truly cold light.
    • The chemistry behind bioluminescence lies in a special molecule called luciferin. When luciferin reacts with another molecule, usually an enzyme called luciferase, in the presence of oxygen, energy is released. This energy takes the form of visible light, creating the characteristic glow. Different luciferins and luciferases determine the emitted light’s colour and intensity.
    • Whilst the light is what we see, bioluminescence is a complex biological process.
    • The materials involved in bioluminescence include:
      • Luciferin: The “fuel” molecule, often a small organic molecule with a specific chemical structure depending on the organism.
      • Luciferase: The “spark,” typically an enzyme that acts as a catalyst, accelerating the reaction between luciferin and oxygen.
      • Oxygen: Essential for most bioluminescent reactions, acting as the final electron acceptor.
    • The reactions involved in bioluminescence include:
      • Activation: Luciferase activates luciferin through various mechanisms, depending on the specific type.
      • Oxidation: Oxygen reacts with the activated luciferin, transferring energy to an excited state.
      • Light emission: As the excited molecule returns to its ground state, energy is released as visible light with a specific wavelength determined by the energy change.

Bipolar cells

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.

  • Bipolar cells are linked to the light-sensitive rod and cone cells in the retina of the human eye.
  • 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.
  • The signals relayed from bipolar cells to ganglion cells include not only the direct responses of bipolar cells to signals resulting from photo-transduction but also responses to signals indirectly received via information from amacrine cells.
  • We could envisage a type of bipolar cell that connects directly from a cone to a ganglion cell and simply conveys information about wavelength. The ganglion cell uses this information to discern whether a specific point in the observed scene is red or green.
  • Not all bipolar cells create synapses directly with a single ganglion cell. Some relay information sampled by various groups of ganglion cells. Others end at different locations within the retina’s complex network of interconnections, enabling them to deliver packets of information to a range of locations and cell types.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

https://en.wikipedia.org/wiki/Retina_bipolar_cell

Blackbody

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 on its temperature.
  • The radiation emitted by a black body is known as blackbody radiation.
  • Blackbody radiation is the type of electromagnetic radiation released by a body in thermodynamic equilibrium with its surroundings. This means that the body emitting the radiation is in a state where there is no net exchange of energy between the body and its environment.
  • 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.
  • The study of blackbody radiation has practical applications in the development and testing of materials for lighting, heating, and thermal imaging equipment.
  • Blackbody radiation is a cornerstone in the study of quantum mechanics.
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References

Bohr model

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.

  • The Bohr model of the atom includes:
    • Central Nucleus: Bohr’s model retained the idea of a central nucleus consisting of positively charged protons and neutral neutrons, around which electrons orbit.
    • Quantized Energy Levels: Unlike the classical model, where electrons could orbit the nucleus at any distance, Bohr proposed that electrons can only occupy certain specific energy levels. These energy levels are quantized, meaning they are discrete and not continuous. Electrons can move between these levels by absorbing or emitting energy in discrete amounts (quanta).
    • Stationary Orbits: Bohr suggested that electrons orbit the nucleus in fixed, circular paths or orbits. Each orbit corresponds to a specific energy level, with the lowest energy level being closest to the nucleus. These orbits are sometimes referred to as “stationary states.”
    • Energy Absorption and Emission: When an electron absorbs energy, it can jump to a higher energy level (further away from the nucleus). Conversely, when it emits energy, it falls back to a lower energy level (closer to the nucleus). The energy emitted or absorbed corresponds to the difference in energy between the initial and final states of the electron.
    • Stability of Orbits: According to the Bohr model, electrons in stable orbits do not emit energy and hence do not spiral into the nucleus. They remain in their orbits until a change in energy causes them to transition to a different orbit.
  • 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.
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References

Brightness

In this resource, 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.

  • In the first instance, 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.
About brightness
  • In this resource, the term brightness is associated with the intensity of light an object such as the Sun or a lightbulb emits.
  • In everyday experience, we often gauge the brightness of a light source subjectively, by comparing it with the brightness of other known light sources.
  • The brightness of a light can also be measured objectively using units like lumens or candela.
  • Light travelling through a vacuum is not visible until it interacts with something such as our eyes or an object that reflects the light towards us, enabling us to perceive its brightness.
  • The perceived brightness of a light source depends on the intensity and wavelength of the light and how the photoreceptive rod and cone cells in the human retina respond.
  • Brightness, when used in this way, is the same as luminance.
  • Luminance is a measure of the amount of light emitted, transmitted, or reflected from a particular area in a specific direction. It is used to quantify the intensity of light that is perceived by the human eye from a particular direction.
  • Our eye’s photoreceptors, especially the rod cells which are more sensitive to light intensity, play a crucial role in our perception of brightness. Rods are more abundant and distributed throughout the retina, and they function mainly in low light conditions to help us perceive the brightness or lightness of an object, but they can’t distinguish colour.
  • On the other hand, our perception of colour is based on how different wavelengths of light stimulate the three types of cone cells in our eyes. These cone cells are sensitive to short (S, which corresponds to blue), medium (M, corresponds to green), and long (L, corresponds to red) wavelengths of light. The combination of signals from these three types of cone cells allows us to perceive a broad spectrum of colours. Colour perception depends not just on the light’s intensity, but on its spectral composition – what mix of wavelengths it contains.
About colour brightness
  • In this resource, the term colour brightness is used to describe how things appear to a human observer in terms of their perception of colour.
  • Colour is what humans perceive in the presence of radiated or reflected light.
  • The brightness of the colour of an object or surface (colour brightness) depends on the wavelengths and intensity of light that illuminate it and the amount of light it reflects.
  • The colour brightness of a transparent or translucent medium may be influenced by the wavelengths and intensity of light that pass through or reflect off it and the amount it transmits or reflects.
  • Colour brightness is frequently influenced by the contrast between how a colour appears to an observer under well-lit conditions and its more subdued appearance when in shadow or under poor illumination.
  • The perception of colour brightness is also influenced by hue, as certain hues appear brighter than others to human observers. For example, a fully saturated yellow may appear relatively brighter than a fully saturated red or blue.
About brightness & colour models
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Brightness: HSB colour model

This entry discusses colour brightness in relation to the HSB colour model, where H represents hue, S represents saturation, and B represents brightness.

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.

About colour brightness
  • In this resource, the term colour brightness is used to describe how things appear to a human observer in terms of their perception of colour.
  • Colour is what humans perceive in the presence of radiated or reflected light.
  • The brightness of the colour of an object or surface (colour brightness) depends on the wavelengths and intensity of light that illuminate it and the amount of light it reflects.
  • The colour brightness of a transparent or translucent medium may be influenced by the wavelengths and intensity of light that pass through or reflect off it and the amount it transmits or reflects.
  • Colour brightness is frequently influenced by the contrast between how a colour appears to an observer under well-lit conditions and its more subdued appearance when in shadow or under poor illumination.
  • The perception of colour brightness is also influenced by hue, as certain hues appear brighter than others to human observers. For example, a fully saturated yellow may appear relatively brighter than a fully saturated red or blue.
About colour models

A colour model derived from colour theory enables a more precise and reproducible method of representing and working with colour.

  • Colour models are a practical application of colour theory that establish terms, definitions, rules or conventions, and systems of notation for encoding colours and their relationships to one another.
  • These days, the most practical colour models are built into applications such as Adobe Creative Cloud and allow seamless digital output to TVs, computers, phones, or printing onto paper and other surfaces.
  • Understanding colour models and utilizing them effectively can contribute to maintaining consistent and accurate colour reproduction across various media.
  • Widely used colour models include:
  • In addition to the colour models mentioned above, numerous other models are used in specific contexts, such as the Lab colour model employed in printing or the LCH colour model used in digital image processing.

A colour model is a framework that allows us to:

  • Make sense of colour in relation to human vision, the surrounding world, and various media and technologies.
  • Understand the relationship between different colours and their properties.
  • Mix specific colours from other colours to achieve predictable and desired results.
  • Specify colours using names, codes, notations, equations, and other forms of representation.
  • Organise and utilize colour for different purposes, such as design, visual arts, or scientific applications.
  • Use colours in consistent and repeatable ways across different platforms and media.
  • Develop systems and rules for blending and using different media, such as light, pigments, or inks.
  • Create colour palettes, define gamuts, and establish colour guides to guide artistic or design decisions.
About brightness & colour models

About colour theory

Colour theories underpin colour management by seeking to explain how human beings perceive colour and establish the rational basis for practical how-to methods for managing colour in different situations.

A system of colour management may be associated with:

Colour theory and human perception

The aspect of colour theory concerned with the human perception of colour aims to answer questions about:

  • How our eyes register colour when exposed to light.
  • The way our eyes and brains work together to produce the complex colour perceptions that make up the visible world.
  • The part of the electromagnetic spectrum that is related to colour and how our eyes respond to different wavelengths of light.
  • The fact that red, green and blue lights combined in different proportions can produce the impression of all the colours of the visible spectrum.
  • The way colours appear in different situations such as in low or bright light and under artificial lighting.
  • Human responses to different combinations of colour such as analogous, complementary and contrasting colours.
  • The differences between the scientific, technical and creative understandings and descriptions of colour.
  • Understanding the differences between:
    • The way our eyes see colour
    • Light and colour in the world around us
    • The colour of opaque objects and surfaces
    • The colour of transparent media
    • Colour on TVs, computers and phone screens
    • Colour in printed images
Colour theory and colour management

The aspect of colour theory concerned with how-to methods for managing colour in different situations aims to answer questions about:

  • The differences between mixing coloured lights, pigment or inks.
  • Mixing and managing ranges (gamuts) of colours in logical, predictable and repeatable ways.
  • Identifying and mixing particular colours in predictable and repeatable ways.
  • Specifying colours using names, codes, notation, equations etc.
  • The difference between additive and subtractive colour mixing.
  • Systems and rules for mixing different and applying them to different materials such as fabrics, interiors and vehicles.
  • Creating colour palettes, gamuts and colour guides.
  • Managing the consistent reproduction of digital colour from start to finish.
Where to find colour theories

About the HSB colour model

The HSB colour model is an additive colour model used to mix light (subtractive colour models are used to mix pigments and inks).

  • The main difference between the HSB colour model and the RGB colour model is how colours are represented and managed in software and applications.
  • The HSB model represents colours based on hue, saturation, and brightness, whereas the RGB model mixes red, green, and blue light to create colours.
  • HSB is popular because it provides a user-friendly way to select and modify colours when using applications like Adobe Creative Cloud for design, photography, or web development.
  • On HSB colour wheels, saturation typically increases from the centre towards the edge.

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.
    • Hue can be measured as a location on an HSB colour wheel and expressed as a degree between 0 and 360.
  • Saturation refers to the vividness of a colour compared to an unsaturated colour.
    • Saturation is measured between a fully saturated colour (100%) and an unsaturated colour (0%)that appear either:
      • Dull and washed out until all colour disappears, leaving only a monochromatic grey tone (0%).
      • Misty or milky the nearer they are to white.
    • On many HSB colour wheels, saturation decreases from the edge to the centre.
  • 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.

About amplitude, brightness, colour brightness and intensity

The terms amplitude, brightness, colour brightness and intensity are easily confused. In this resource:

Amplitude
Brightness
  • Brightness refers to a property of light, to how strong a light source or light reflected off an object appears to be.
  • Brightness is related to how things appear from the point of view of an observer.
    • When something appears bright it seems to radiate or reflect more light or colour than something else.
    • Brightness may refer to a light source, an object, a surface, transparent or translucent medium.
    • The brightness of light depends on the intensity or the amount of light an object emits( eg. the Sun or a lightbulb).
    • The brightness of the colour of an object or surface depends on the intensity of light that falls on it and the amount it reflects.
    • The brightness of the colour of a transparent or translucent medium depends on the intensity of light that falls on it and the amount it transmits.
    • Because brightness is related to intensity, it is related to the amplitude of electromagnetic waves.
    • Brightness is influenced by the way the human eye responds to the colours associated with different wavelengths of light. For example, yellow appears relatively brighter than reds or blues to an observer.
Colour Brightness
  •  Colour brightness refers to how colours appear to a human observer in terms of the lightness or darkness of colours.

So colour brightness can refer to the difference between how a colour appears to an observer in well-lit conditions and its subdued appearance when in shadow or when poorly illuminated.

  • In a general sense, brightness is an attribute of visual perception and produces the impression that something is radiating or reflecting light and/or colour.
  • Colour brightness increases as lighting conditions improve, whilst the vitality of colours decreases when a surface is poorly lit.
  • Optical factors affecting colour brightness include:
  • Material properties affecting the colour brightness of a medium, object or surface include:
    • Chemical composition
    • Three-dimensional form
    • Texture
    • Reflectance
  • Perceptual factors affecting colour brightness include:
Intensity
    • Intensity refers to the amount of light produced by a light source or the amount of light that falls on a particular area of the object.
    • So intensity measures the energy carried by a light wave or stream of photons:
      • When light is modelled as a wave, intensity is directly related to amplitude.
      • When light is modelled as a particle, intensity is directly related to the number of photons present at any given point in time.
      • Light intensity falls exponentially as the distance from a point light source increases.
      • Light intensity at any given distance from a light source is directly related to its power per unit area (when the area is measured on a plane perpendicular to the direction of propagation of light).
      • The power of a light source describes the rate at which light energy is emitted and is measured in watts.
      • The intensity of light is measured in watts per square meter (W/m2).
      • Cameras use a light meter to measure the light intensity within an environment or reflected off a surface.
About colour brightness & light intensity
  • The perception of colour depends on the wavelengths that reach an observer’s eyes. Red has a longer wavelength, while violet has a shorter wavelength.
  • Any colour (e.g. red, magenta, or violet) can be defined by its hue, saturation, and brightness.
  • Saturated colours are produced by a single wavelength of light or a narrow band of wavelengths.
  • The brightness of a colour depends on the intensity of the light emitted by a light source (e.g., a coloured light bulb) and the amount of light reflected from a coloured surface.
    • So, for example, the texture of a surface can affect brightness even when the intensity of the light source remains constant.
  • The intensity of light, along with factors such as phase and interference, are directly related to the amplitude of an electromagnetic wave.
  • Amplitude measures the height of light waves from the centre-line of a waveform to its crest or to a corresponding trough.
  • Colour brightness, light intensity, and the amplitude of a light wave can all be thought of in terms of the number of photons that strike the eye of an observer.
    • Therefore, increasing the amplitude of a wavelength of light will increase the number of photons falling on an object, making it appear brighter to an observer.
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References
  • 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.
  • Follow this link for a discussion of colour brightness in relation to the HSB colour model, where H represents hue, S represents saturation, and B represents brightness.

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.
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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.
Properties of electric charge
  • Like charges repel, opposite charges attract: Particles and objects with the same type of charge repel each other, while objects with opposite charges attract each other.
  • Electrostatic force: The force between charged objects is called the electrostatic force, and it’s governed by Coulomb’s law.
  • Conservation of charge: The total electric charge in an isolated system is always conserved. It cannot be created or destroyed, only transferred between particles or regions.
  • This means, for example, that even though electrons can move between energy levels within an atom, the total number of protons (positive charges) and electrons (negative charges) remains constant, upholding the principle of conservation of charge.
Examples of charge
  • Static electricity: Rubbing a balloon on your hair transfers electrons, creating a static charge that can make hair stand on end or attract small objects.
  • Electric current: The flow of electrons through a conductor, like a wire, creates an electric current, which can be used to power our devices.
  • Lightning: Lightening is a dramatic example of charge-discharge in nature, caused by the buildup of static electricity in clouds.
About electric charge and energy in an atom
  • An atom has a set number of particles that determines what kind of element they are.
  • Each element has a specific number of protons (positive charge) in its nucleus. It is this number that defines an element and cannot change in everyday situations.
  • Protons in the nucleus of an atom are very stable and don’t typically move around or get created or destroyed. There are a fixed number and they maintain their positive charge within the atom.
  • Electrons (negative charge) surround the nucleus, with the number typically matching the number of protons to create a neutral atom.
  • Regardless of their energy level, an atom retains the same number of electrons it started with.
  • If an atom loses or gains an electron, it is no longer considered the same element and becomes an ion.
  • The movement of electrons within an atom doesn’t change their total charge because the number of protons and electrons remains constant. However, the movement of electrons does affect the amount of energy within the atom.
  • Electrons in an atom change energy levels as they gain and lose energy.
  • So when an electron absorbs energy (such as visible light), it jumps to a higher energy level further away from the nucleus. However, the electron itself remains negatively charged, it just occupies a different position within the atom.
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References
  • 1
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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 other 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.
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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.

The most common types of chemical bonds are:

Covalent Bonds
  • Covalent bonds occur when electrons are shared between two atoms. The shared electrons are attracted to the protons of both nuclei, which keeps the atoms bonded together.
Ionic Bonds
  • Ionic bonds occur when one atom completely transfers one or more electrons to another atom. This creates ions, with the atom that loses electrons becoming a positively charged ion and the atom that gains electrons becoming a negatively charged ion. The attraction between these opposite charges keeps the ions bonded together.
Metallic Bonds
  • Metallic bonds are found in metals; they consist of the electrostatic attractive force between the conduction electrons, in what is known as an electron cloud, and the positively charged metal ions. These bonds allow for characteristics such as high melting points, malleability, and conductivity.
Hydrogen Bonds
  • Hydrogen bonds are a type of dipole-dipole interaction that happens when a hydrogen atom bonded to a strongly electronegative atom (like nitrogen, oxygen, or fluorine) is also attracted to another electronegative atom in the same or another molecule.
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References

https://en.wikipedia.org/wiki/Chemical_bond

Chemiluminescence

Chemiluminescence is a type of luminescence where light is emitted as a direct result of a chemical reaction. Unlike other luminescence mechanisms that might involve external energy sources such as light or electricity, chemiluminescence relies solely on the chemical energy stored within the reacting molecules.

Key features of chemiluminescence
  • Energy source: Chemical reactions release energy in various forms, including light. This energy excites electrons in chemiluminescence, leading them to higher energy levels.
  • Electron transitions: Excited electrons return to their ground state, releasing excess energy as light. This emission determines the light’s colour.
  • Examples: Glow sticks, luminous fungi, and deep-sea organisms.
  • Variations: Emitted light characteristics (intensity, colour, duration) depend on the specific chemical reaction.
Mechanisms of chemiluminescence
  • Electron transfer: One reactant loses an electron (oxidation), the other gains an electron (reduction). This transfer can excite an electron in the accepting molecule, leading to light emission.
  • Excited intermediates: Some reactions create intermediate molecules in excited states. These release excess energy as light when returning to their ground state.
  • Free radicals: Highly reactive free radicals can undergo rearrangements or reactions with other molecules, releasing energy as light.
Applications
  • Chemical analysis: Detecting specific substances based on unique light emission patterns of certain reactions.
  • Biological research: Studying biological processes involving chemiluminescent molecules in organisms.
  • Safety devices: Light sticks for emergency lighting or marking locations use chemiluminescent reactions.
Light sources
Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Adhesive tape peeling
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms		Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars		Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles		Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests		Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs		ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks		Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters		Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights		ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
Pressure-sensitive adhesives		TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests		PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

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References

Summary

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 denotes the process of separation according to colour.
  • 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.
  • Keep in mind that wavelength is a characteristic of electromagnetic radiation, whilst colour is an aspect of visual perception.
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References

https://en.wikipedia.org/wiki/Dispersion_(optics)

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
Chromaticity diagram
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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.
Chromaticity diagram description
  • The colours in a chromaticity diagram appear on a horseshoe-shaped block positioned between two axes and taken as a whole, represents the entire range of colours a person with average eyesight perceives.
  • Each point within the horseshoe signifies the position of a unique spectral colour in terms of its hue and saturation.
  • In theory, a high-quality HD display can show a chromaticity diagram containing over 16 million colours.
  • The scales on the x and y axes can be used to plot the component RGB values associated with any colour.
    • As the values increase along the x-axis, the amount of blue in a colour decreases as red increases.
    • As the values increase along the y-axis, the amount of blue in a colour decreases as green increases.
    • As values increase along both the x and y axis the diagram locates every possible colour.
  • Fully saturated colours appear along the boundary of the horseshoe and every pixel corresponds with the wavelength of a single spectral colour measured in nanometres.
  • The fully saturated spectral colours along the boundary become less saturated towards the centre of the diagram.
  • The circle in the centre, the white point, indicates the point where amounts of red, green and blue add together to produce a neutral white.
  • The white point represents the appearance of natural light at midday.
  • Two-dimensional chromaticity diagrams don’t show the z-axis which corresponds with colour brightness.

Chromaticity diagram

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Chromophore

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

  • Things appear to have colour because they absorb certain wavelengths of light while reflect 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.
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References

https://en.wikipedia.org/wiki/Chromophore

  • The chromophore is the part of a molecule that produces its colour.
  • Things appear to have colour because they absorb certain wavelengths of light while reflect 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.

CIE

The International Commission on Illumination (usually abbreviated CIE for its French name, Commission internationale de l’éclairage) is the international authority on light, illumination, colour, and colour spaces. It was established in 1913 as a successor to the Commission Internationale de Photométrie, which was founded in 1900, and is today based in Vienna, Austria.

CIE 1931 XYZ was the first attempt to produce a colour space based on measurements of human colour perception and the basis for almost all other colour spaces.

In 1976, the commission developed the CIELAB and CIELUV colour spaces, which are widely used today.

CIE (1931) XYZ colour space

A color space is a specific system used to represent and categorize colours. It’s essentially a way to define and organize the range of colours that can be perceived by the human eye or captured by a device like a camera or monitor.

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 able to systematise the relationship between colour stimuli and human colour perception.
  • In an experimental situation, the CIE XYZ colour space is able to 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.

The CIE XYZ colour space serves as a standard reference and underpins more recent colour spaces such as:

  • CIELUV 1976 –  a modification of CIE 1931 XYZ used to display additive mixtures of light more conveniently.
  • CIELAB 1976 –  a more perceptually linear colour space. Perceptually linear means that changes in colour values are directly related to changes in colour appearance.  CIELAB is commonly used for surface colours, but not for mixtures of light.
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References
  • 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 is able to 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.

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.
Core concepts of classical electromagnetism
  • Charged Particles (Matter): These are particles that have an electric charge, either positive (protons) or negative (electrons). They are the sources of electric and magnetic fields and are affected by these fields.
  • Electromagnetic Force: This force is a fundamental interaction between charged particles. It can be attractive or repulsive, depending on the sign of the charges.
  • Electromagnetic Fields: These are regions where electric and magnetic forces are experienced due to the presence of charged particles. Electromagnetic fields carry energy and can exert forces on other charged particles.
Everyday examples of Maxwell’s electromagnetism
  • When you turn on a light switch, the electric current in the filament of the light bulb produces a magnetic field. This in turn produces an electric field causing the filament to glow white hot.
  • When you listen to the radio, the electromagnetic waves from the radio station interact with the antenna on your radio to produce an electric current. This electric current is then amplified and converted into sound, which you can hear through the speakers on your radio.
  • When you use a microwave oven to heat food, the electromagnetic waves from the microwave oven interact with the water molecules in the food. This causes the water molecules to vibrate, which heats up the food.
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References
  • 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.