A rainbow is an optical effect

A rainbow is an optical effect, a trick of the light, caused by the behaviour of light waves travelling through transparent water droplets towards an observer.

  • Sunlight and raindrops are always present when a rainbow appears but without an observer, there is nothing, because eyes are needed to produce the visual experience.
  • A rainbow isn’t an object in the sense that we understand physical things in the world around us. A rainbow is simply light caught up in raindrops.
  • A rainbow has no fixed location. Where rainbows appear depends on where the observer is standing, the position of the Sun and where rain is falling.
  • The exact paths of light through raindrops is so critical to the formation of rainbows that when two observers stand together their rainbows are produced by different sets of raindrops.

About this dictionary

This DICTIONARY OF LIGHT, COLOUR & VISION contains a vocabulary of closely interrelated terms that underpin all the resources you will find here at lightcolourvision.org.

  • Each term has its own page in the DICTIONARY and starts with a DEFINITION.
  • Bullet points follow that provide both context and detail.
  • Links embedded in the text throughout the site (highlighted in blue) take you directly to DICTIONARY entries.
  • Shorter SUMMARIES of terms appear on DIAGRAM PAGES under the heading SOME KEY TERMS. These entries strip definitions back to basics and can be viewed without leaving the page.
Why a dictionary of light, colour & vision
  • One of the practical objectives of this website is to make the connections between the topics of light, colour and vision accessible to students and researchers of all ages.
  • Our DICTIONARY aims to avoid a problem faced by websites such as Wikipedia where articles are often composed by contributors with narrow specialisation and their own topic-specific vocabulary.
  • The layout of the DICTIONARY also aims to avoid situations where a single unknown word or phrase makes it difficult, if not impossible, for our visitors to find the information they need (as explained below).
Terms, definitions and explanations
  • All the terms we have selected for the DICTIONARY are widely used and are applied consistently across the topics of light, colour and vision.
  • The aim is to avoid definitions and explanations with different meanings in different fields.
  • As far as possible definitions contain no more than two short sentences.
  • The explanations that follow each definition are arranged as short bullet points that avoid paragraphs of information completely.
  • Each bullet makes a stand-alone point and is intended to deal with a single piece of information that we believe is likely to be important to our readership.
  • The writing style across all terms aims to be clear, accessible and engaging.
  • The idea is to enable our visitors to find and digest information quickly and to confirm facts one at a time.
  • Because our readership and their concerns are diverse, bullet points sometimes provide different perspectives on a single term or topic.

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Absorption

When light strikes an object, some wavelengths are absorbed and their energy is converted to heat, others undergo reflection or transmission.

  • When light is absorbed by an object or medium, its energy excites electrons, which emit heat.
  • Absorption of a particular wavelength of light by a material occurs when the frequency of the wave matches the resonant frequency of electrons orbiting atomic nuclei in the material.
  • The opposite of absorption is transmission which refers to the process of electromagnetic radiation passing through a medium. When electromagnetic waves move through a material without being absorbed or reflected, we say they are transmitted. If no radiation is reflected or absorbed at all, the material achieves 100% transmission.
  • Electrons selectively absorb photons whose frequencies match their resonant frequencies.
  • Resonant frequency refers to the frequency at which electrons are most efficiently excited by absorbed light, leading to the emission of heat.
  • As electrons absorb energy from photons, they vibrate more vigorously, leading to collisions between atoms and the production of heat.
  • When light is reflected from a surface, it bounces off at the same wavelength with little or no change in energy.
  • When light is absorbed by an object or medium, its energy excites electrons, causing them to vibrate more vigorously and to collide with other atoms, which in turn produces heat.
  • Materials selectively absorb photons whose frequencies match their own frequencies.
  • Absorption of light occurs when the frequency of the wave matches the frequency of electrons orbiting atomic nuclei in a material.
  • Reflected light bounces off a surface at the same wavelength with little or no change in energy.

Absorption

Accommodation

Accommodation refers to the way the lenses inside our eyes accommodate for the fact that objects of interest may be close to or at a distance. Sharp images on the retina are the result of modifying the focal length of each lens.

  • The lens in each eye is located just behind the pupil. The shape of the lens is controlled by the ciliary muscle which forms a ring of flexible tissue around the edge of each lens.
  • The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance between the centre of the lens and the retina always remains the same.
  • The focal length of a lens is the distance at which it brings parallel rays of light into focus on the retina.
    • A lens with a shorter focal length brings objects closer to the eye into focus and has a wider field of view.
    • Lens with a longer focal length bring objects at a greater distance from the eye into focus and has a narrower field of view.
  • Because the image distance is fixed in human eyes, they accommodate for this by using the ciliary muscle to alter the focal length of the lens.
    • To accommodate nearby objects the ciliary muscle contracts, making the lens more convex with a shorter focal length.
    • To accommodate distant objects the ciliary muscle relaxes, causing the lens to adopt a flatter and less convex shape with a longer focal length.
  • Accommodation refers to the way the lenses inside our eyes accommodate for the fact that objects of interest may be close to or at a distance. Sharp images on the retina are the result of modifying the focal length of each lens.
  • The focal length of a lens is the distance at which it brings parallel rays of light into focus on the retina.
    • A lens with a shorter focal length brings objects closer to the eye into focus and has a wider field of view.
    • Lens with a longer focal length bring objects at a greater distance from the eye into focus and has a narrower field of view.

Accommodation

Accommodation

The distance between the retina (the detector) and the cornea (the refractor) is fixed in the human eyeball. The eye must be able to alter the focal length of the lens in order to accurately focus images of both nearby and far away objects on the retinal surface. This is achieved by small muscles that alter the shape of the lens. The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance remains constant.

The ability of the eye to adjust its focal length is known as accommodation. The eye accommodates by assuming a lens shape that has a shorter focal length for nearby objects in which case the ciliary muscles squeeze the lens into a more convex shape. For distant objects, the ciliary muscles relax, and the lens adopts a flatter form with a longer focal length.

Accommodation

  • Accommodation refers to the way the lenses inside our eyes accommodate for the fact that objects of interest may be close to or at a distance. Sharp images on the retina are the result of modifying the focal length of each lens.
  • The focal length of a lens is the distance at which it brings parallel rays of light into focus on the retina.
    • A lens with a shorter focal length brings objects closer to the eye into focus and has a wider field of view.
    • A lens with a longer focal length brings objects at a greater distance from the eye into focus and has a narrower field of view.

Achromatic

Achromatic means without colour so refers to surfaces or objects that appear white, grey or black. Achromatic colours lack hue or saturation but can be described in terms of their brightness.

  • Near-neutral colours such as tints, shades or tones are often considered achromatic.
  • The lightest shades of pastel colours can be nearly achromatic, but they may still exhibit a subtle hint of hue.
  • Deep shadows and the world as it appears at night can appear achromatic, but may still retain some colour.
  • When mixing paint, achromatic colours are produced by adding black and/or white until the original colour is fully desaturated.
  • Achromatic colours are produced on digital screens by mixing red, green and blue light in equal proportions.
  • The RGB colour model produces achromatic colours when all three components of a colour have the same value. So the RGB colour values R = 128, G = 128, B = 128 produce a middle grey, which is an achromatic colour.
  • The way achromatic colours appear to an observer often depends on adjacent more saturated colours. So, next to a bright red couch, a grey wall may appear to have a slightly greenish tint due to the phenomenon known as simultaneous contrast.
  • Achromatic means without colour so refers to surfaces or objects that appear white, grey or black. Achromatic colours lack hue or saturation but can be described in terms of their brightness.
  • When mixing paint, achromatic colours are produced by adding black and/or white until the original colour is fully desaturated.
  • Achromatic colours are produced on digital screens by mixing red, green and blue light in equal proportions.

Additive & RGB colour

About additive & RGB colour

The RGB colour model used by TV, computer and phone screens involves additive colour mixing.

  • The RGB colour model produces all the colours seen by an observer on TV, computer and phone screens by creating arrays of red, green and blue pixels (picture elements) in different proportions.
  • Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours alone can produce any conceivable colour if blended in the correct proportion.
  • Different colours are produced by varying the brightness of the component colours between completely off and fully on.
  • When fully saturated red, green and blue primary colours are mixed in equal amounts, they produce white.
  • A fully saturated hue is produced by a single wavelength (or narrow band of wavelengths) of light.
  • When any two fully saturated additive primary colours are mixed, they produce a secondary colour: yellow, cyan or magenta.
  • Some implementations of RGB colour models can produce millions of colours by varying the brightness of each of the three primary colours.
  • The additive RGB colour model cannot be used for mixing pigments such as paints, inks, dyes or powders.
  • The RGB colour model does not define the exact hue of the three primary colours so the choice of wavelengths for each primary colour is important if it is to be used as part of a colour-managed workflow.
  • The RGB colour model can be made device-independent by specifying a colour profile such as sRGB or Adobe RGB (1998) which ensures consistent results regardless of the device used to output an image.

Additive & subtractive colour

Additive colour is shorthand for the additive mixing of wavelengths of light to produce colour. The method involves mixing wavelengths corresponding with primary colours at varying intensities and projecting them onto a surface or screen. When seen by an observer, light enters and stimulates the eyes and, depending on the intensity of the signal on each channel, produces the visual impression of a predicted colour.

  • Whilst additive colour mixing is the method used to combine wavelengths of light, subtractive colour mixing is the method used with dyes, inks and pigments.
  • An additive approach to colour mixing is used in the case of the emission of light by light-emitting diodes (or similar light sources) embedded into the screens of mobile phones, computers and televisions etc.
  • An additive approach to colour mixing is also used with digital projectors. In this case, sufficient light must be produced on each channel to form intense images when focused onto a screen across a room.
  • RGB colour is one of the additive colour models that combine wavelengths of light corresponding with red, green and blue primary colours to produce other colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce other colours. Red green and blue are often described as being components of the resulting colour.
  • Different colours are produced by varying the intensity of the component colours between fully off and fully on.
  • When fully saturated red, green and blue primary colours are combined, they produce white.
  • When any two fully saturated additive primaries are combined, they produce a secondary colour: yellow, cyan and magenta.
  • Some RGB colour models can produce over 16 million colours by fine-tuning the intensity of each of the three primary colours.
  • The additive RGB colour model cannot be used for mixing different colours of pigments, paints, inks, dyes or powders. To combine these colourants subtractive colour models are used.

Additive & subtractive colour models

About additive and subtractive colour models

There are two main types of colour models, additive and subtractive.

Additive Colour Models
Subtractive Colour Models
  • Subtractive colour models are used when working with pigments, inks and dyes.
  • The primary colours for most subtractive colour models are cyan, magenta, and yellow (CMY).
  • When combined cyan, magenta, and yellow produce black.
  • The subtractive CMY colour model and CMYK colour model are central to printing technologies.
  • In practice, the CMY colours often can’t produce a perfect black when mixed due to impurities in the pigments or inks, so a fourth ‘Key’ component (represented as K) is often used in printing to produce a true black.

Additive colour

Additive colour model

An additive colour model explains how different coloured lights (such as LEDs or beams of light) are mixed to produce other colours. The RGB colour model and HSB colour model are examples of additive colour models.

  • Additive colour refers to the methods used and effects produced by combining or mixing different wavelengths of light.
  • Additive colour models such as the RGB colour model and HSB colour model can produce vast ranges of colours by combining red, green, and blue lights in varying proportions.
  • An additive approach to colour is used to achieve precise control over the appearance of colours on digital screens of TVs, computers, and phones.
  • Subtractive colour models such as the CMY colour model provide methods for mixing pigments such as dyes, inks, or paints to produce different colours.
  • Both additive and subtractive colour models rely on mixing primary colours in different proportions:
    • CMY refers to the primary colours cyan (C), magenta (M) and yellow (Y).
    • CMYK refers to the primary colours cyan, magenta and yellow plus black (K).
    • RYB refers to the primary colours red (R), yellow (Y) and blue (B).
  • Both additive and subtractive colour models can be studied and understood by exploring colour wheels.
About additive & RGB colour

The RGB colour model used by TV, computer and phone screens involves additive colour mixing.

  • The RGB colour model produces all the colours seen by an observer on TV, computer and phone screens by creating arrays of red, green and blue pixels (picture elements) in different proportions.
  • Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours alone can produce any conceivable colour if blended in the correct proportion.
  • Different colours are produced by varying the brightness of the component colours between completely off and fully on.
  • When fully saturated red, green and blue primary colours are mixed in equal amounts, they produce white.
  • A fully saturated hue is produced by a single wavelength (or narrow band of wavelengths) of light.
  • When any two fully saturated additive primary colours are mixed, they produce a secondary colour: yellow, cyan or magenta.
  • Some implementations of RGB colour models can produce millions of colours by varying the brightness of each of the three primary colours.
  • The additive RGB colour model cannot be used for mixing pigments such as paints, inks, dyes or powders.
  • The RGB colour model does not define the exact hue of the three primary colours so the choice of wavelengths for each primary colour is important if it is to be used as part of a colour-managed workflow.
  • The RGB colour model can be made device-independent by specifying a colour profile such as sRGB or Adobe RGB (1998) which ensures consistent results regardless of the device used to output an image.
  • An additive colour model explains how different coloured lights (such as LEDs or beams of light) are mixed to produce other colours.
  • Additive colour refers to the methods used and effects produced by combining or mixing different wavelengths of light.
  • The RGB colour model and HSB colour model are examples of additive colour models.
  • Additive colour models such as the RGB colour model and HSB colour model can produce vast ranges of colours by combining red, green, and blue lights in varying proportions.
  • An additive approach to colour is used to achieve precise control over the appearance of colours on digital screens of TVs, computers, and phones.

Adobe RGB (1998) colour space

  • The general purpose of a colour space is to determine the range of colours available within a specific workflow and may be determined by a user or programmatically.
  • The Adobe RGB (1998) colour space aims to ensure the optimal range of colours available within the RGB colour model are accurately reproduced when output to digital displays or printers.
  • In a digital environment, the aim is to ensure that a selected range of colours appears consistent throughout a workflow and that the desired range of colours is successfully reproduced at the end of the process.

Adobe RGB colour space

The Adobe RGB (1998) colour space was developed by Adobe Systems. It aims to ensure the optimal range of colours available within the RGB colour model are accurately reproduced when output to a digital displays or printers.

  • The general purpose of a colour space is to determine the range of colours available within a specific workflow and may be determined by a user or programmatically.
  • The purpose of RGB (1998) was to improve on the gamut of colours that can be produced by the earlier sRGB colour space, primarily in the reproduction of cyan-green hues.
    • It can reproduce approximately 35% more colours compared to sRGB in ideal conditions.
    • It is estimated to cover approximately 50% to 70% of the colours perceived by the human eye in ideal conditions.
  • In a digital environment, the aim is to ensure that a selected range of colours appears consistent throughout a workflow and that the desired range of colours is successfully reproduced at the end of the process.
About colour spaces & examples
  • A colour space aims to accurately define the relationship between any selected colour within a colour model and how it will appear when it is reproduced by a specific device such as a digital display, printer or paint mixing machine.
  • When an artist selects a limited number of tubes of oil paint to add to a palette, they are already working within the RYB subtractive colour model and establishing the colour space in which they plan to work.
  • A colour space may aim to limit the number of colours or establish the widest possible gamut of reproducible colours.
  • Digital colour spaces are commonly used to accurately set the range of colours that can be output to and then displayed by digital screens and printers.
  • When a colour space is to be matched with a specific digital device such as a projector or printer, a colour profile is loaded along with the image file to ensure accurate colour reproduction.
  • A colour profile is a program that enables a piece of equipment, such as a digital printer, to know how to handle and process the information it receives, ensuring it can produce the intended colour output accurately.

Examples of colour spaces include:

RGB (Red, Green, Blue) Color Space
CMYK (Cyan, Magenta, Yellow, Key/Black) Color Space
  • CMYK is a subtractive colour model used in printing and design. It defines colours by subtracting varying amounts of cyan, magenta, yellow, and black ink from a white paper background. CMYK is used to achieve a wide range of colours on printed materials.
RYB (Red, Yellow, Blue) Color Space
  • RYB is an older subtractive colour model primarily used in traditional art and paint mixing. It consists of three primary colours: red, yellow, and blue. Mixing these colours creates secondary colours, such as orange, green, and violet. RYB is not used in modern digital design.
LAB Color Space
  • LAB is a device-independent colour space that represents colours in a way that is closer to human perception. It separates colour information into three channels: L (lightness), A (green to red), and B (blue to yellow). LAB is used in colour management and as an intermediate space when converting between different colour models.
HSB/HSV (Hue, Saturation, Brightness/Value) Color Space
  • HSB/HSV is a cylindrical colour model that represents colours based on three parameters: hue (the type of colour), saturation (the purity of the colour), and brightness/value (the intensity of the colour). It is often used in computer graphics and design software.
XYZ Color Space
  • XYZ is a CIE (Commission Internationale de l’Eclairage) standardized colour space that serves as a reference for defining other colour spaces. It is based on human vision and designed to be perceptually uniform. XYZ is used in various colour-related calculations and conversions.
Pantone Color Space
    • The Pantone colour system is widely used in the printing and design industries. It provides a standardized set of colours represented by specific codes. Each colour swatch is carefully defined to ensure consistency in printing and reproducing colours accurately.
  • The general purpose of a colour space is to determine the range of colours available within a specific workflow and may be determined by a user or programmatically.
  • The Adobe RGB (1998) colour space  aims to ensure the optimal range of colours available within the RGB colour model are accurately reproduced when output to a digital displays or printers.
  • In a digital environment, the aim is to ensure that a selected range of colours appears consistent throughout a workflow and that the desired range of colours is successfully reproduced at the end of the process.

Adobe RGB, sRGB & ProPhoto

About Adobe RGB, ProPhoto RGB & sRGB

The most common colour profiles in photography are sRGB, Adobe RGB (1998), and ProPhoto RGB.

  • Adobe RGB, developed in 1998, consists of the same red green blue colours as sRGB but the colour space has a larger gamut.
    • It was developed to communicate with standard CMYK multi-function and inkjet printers and is commonly used for printing on fine art papers.
    • When the RGB colour model is used on a modern computer screen, the Adobe RGB (1998) colour space aims to reproduce roughly 50% of the range of colours that an observer is capable of seeing in ideal conditions.
    • The Adobe RGB (1998) colour space was developed to improve on the gamut of colours that could be produced by the earlier sRGB colour space, primarily in the reproduction of cyan-green hues.
  • sRGB stands for standard red green blue and has the smallest colour space.
    • It was developed by HP and Microsoft in 1996 for use with monitors, printers, and the World Wide Web.
    • It is the most commonly used colour profile today because of its consistent reproduction of colours across different platforms.
  • ProPhoto RGB has the largest colour space with a gamut that covers a significant part of the perceptual colour space of the human eye.
    • ProPhoto RGB is used in high-end photography and editing workflows to preserve a wider range of colours and maintain the quality of the original image during processing.

Airglow

Airglow is a faint, continual emission of light originating from the Earth’s upper atmosphere, typically between 80 and 400 kilometres in altitude. While often mistaken for distant starlight, it forms a distinct phenomenon with unique characteristics and scientific significance.

  • Emission Process: Airglow primarily results from chemiluminescenceSolar radiation ionizes atmospheric molecules like oxygen, nitrogen, and sodium during the day. These excited molecules later recombine with other particles at night, releasing energy as light in specific wavelengths.
  • Spectral Colours: Different molecules emit light at characteristic colours:
    • Green: Primary emission from excited oxygen atoms.
    • Red: Mainly from sodium atoms, contributing to the reddish band above the horizon.
    • Blue and violet: Emissions from hydroxyl (OH) and nitric oxide (NO) molecules.
  • Visibility and Variations: Airglow intensity varies due to altitude, wavelength, and location. Magnetic storms can enhance brightness, creating spectacular displays. Astronauts observe airglow as a luminous band encircling Earth.
References
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  • Airglow is a faint, continual emission of light originating from the Earth’s upper atmosphere, typically between 80 and 400 kilometres in altitude. While often mistaken for distant starlight, it forms a distinct phenomenon with unique characteristics and scientific significance.
  • Emission Process: Airglow primarily results from chemiluminescenceSolar radiation ionizes atmospheric molecules like oxygen, nitrogen, and sodium during the day. These excited molecules later recombine with other particles at night, releasing energy as light in specific wavelengths.
  • Spectral Colours: Different molecules emit light at characteristic colours:
    • Green: Primary emission from excited oxygen atoms.
    • Red: Mainly from sodium atoms, contributing to the reddish band above the horizon.
    • Blue and violet: Emissions from hydroxyl (OH) and nitric oxide (NO) molecules.
  • Visibility and Variations: Airglow intensity varies due to altitude, wavelength, and location. Magnetic storms can enhance brightness, creating spectacular displays. Astronauts observe airglow as a luminous band encircling Earth.

Alexander’s band

Alexander’s band (Alexander’s dark band) is an optical effect associated with rainbows. The term refers to the area between primary and secondary bows that often appears to be noticeably darker to an observer than the rest of the sky.

  • The areas of sky around a rainbow may appear blue or grey depending on weather conditions and the amount of cloud in the sky. But these areas outside, inside and between primary and secondary rainbows tend to appear tonally different from one another:
  • Alexander’s band can be explained by the fact that fewer photons are directed from this area of the sky toward an observer.
  • The raindrops that form a primary rainbow all direct exiting light downwards towards an observer so away from Alexander’s band.
  • The raindrops that form a secondary bow all direct light upwards, so away from Alexander’s band, before a second internal reflection directs light downwards towards an observer.
  • Alexander’s band is named after Alexander of Aphrodisias, an ancient Greek philosopher who commented on the effect in his writing.

Alexander’s band

Alexander’s band, also known as Alexander’s dark band, is an optical phenomenon observed in rainbows. It refers to the region between the primary and secondary bows, which often appears noticeably darker to an observer compared to the rest of the sky.

  • The areas of sky around a rainbow may appear blue or grey depending on weather conditions and cloud cover. However, these areas outside, inside, and between the primary and secondary rainbows tend to have distinct tonal differences from one another:
  • Alexander’s band can be explained by the fact that fewer photons are directed from this specific area of the sky toward an observer.
  • The raindrops that form a primary rainbow all direct exiting light downwards towards an observer so away from Alexander’s band.
  • The raindrops that form a secondary bow all direct light upwards, so away from Alexander’s band, before a second internal reflection directs light downwards towards an observer.
  • Alexander’s band is named after Alexander of Aphrodisias, an ancient Greek philosopher who made observations and comments about this phenomenon in his writings.
Related diagrams

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

  • Alexander’s band, also known as Alexander’s dark band, is an optical phenomenon observed in rainbows. It refers to the region between the primary and secondary bows, which often appears noticeably darker to an observer compared to the rest of the sky.
  • The areas of sky around a rainbow may appear blue or grey depending on weather conditions and cloud cover. However, these areas outside, inside, and between the primary and secondary rainbows tend to have distinct tonal differences from one another:
    • The area inside the arcs of a primary rainbow always appears tonally lighter than the surrounding sky.
    • The area outside primary and secondary rainbows appears darker.
    • The area between primary and secondary rainbows appears the darkest – this is Alexander’s band.
  • Alexander’s band can be explained by the fact that fewer photons are directed from this specific area of the sky toward an observer.