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 is transferred to electrons and emitted as heat.
  • Absorption of a particular wavelength of light into a material takes place when the frequency of the wave matches the frequency of electrons orbiting atomic nuclei.
  • Electrons selectively absorb photons with matching frequencies.
  • As electrons orbiting atomic nuclei absorb energy, they vibrate more vigorously causing atoms to collide with one which produces heat.
  • When light is reflected off a surface it bounces off at the same wavelength with little or no change in energy.

Accommodation

Accommodation refers to the way our eyes keep things in focus by changing the shape of the lens in each eye. The result is sharp images of the world regardless of whether things are close by or in the distance.

  • The lens in each eye is located just behind the pupil. The lens shape is controlled by ciliary muscle.
  • The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance always remains the same.
  • Image distance is measured between the retina (the light-sensitive surface at the back of the eye) and the centre of the lens and is fixed in the case of the human eyeball.
  • Because the image distance is fixed, our eyes accommodate for this by using the ciliary muscle to alter the focal length of the lens. This enables images of both nearby and far away objects to be brought into sharp focus on the retinal surface.
  • Ciliary muscle forms a ring of flexible tissue around the edge of each lens.
  • Our eyes accommodate nearby objects by forming each lens into a shape with a shorter focal length. In this case, the ciliary muscle squeezes the lens into a more convex form.
  • Our eyes accommodate distant objects, by relaxing the ciliary muscle, causing the lens to adopt a flatter and less convex shape with a longer focal length.

Summary

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.

Achromatic

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

  • Near-neutral colours such as tints or dark tones are achromatic.
  • The lightest shades of pastel colours are almost achromatic.
  • Deep shadows and the world as it appears in darkness are almost achromatic.
  • When mixing paint, achromatic hues are produced by adding black and/or white until the original colour almost disappears.
  • Achromatic colours are produced on digital screens by mixing red, green and blue light in equal proportions. The RGB colour model produces achromatic hues when all three components of a colour have the same value. So the RGB colour values R = 128, G = 128, B = 128 together produce a middle grey.
  • The way achromatic hues appear to an observer often depends on adjacent more saturated colours. So, next to a bright red couch, a grey wall will appear distinctly greenish – green and red being complementary colours.

Additive colour

Additive colour refers to the way any two or more wavelengths of light can be combined to produce another colour.  The RGB colour model, HSB colour model and Spectral colour model use additive methods to produce systematic ranges of colour.

About additive colour and the RGB colour model

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 simply by combining the light emitted by arrays of red, green and blue pixels (picture elements) in different proportions.

  • RGB colour is an additive colour model that combines wavelengths of light corresponding with red, green and blue primary colours to produce all other colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours can produce any other colour if mixed in the right proportion.
  • 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 in equal amounts, they produce white.
  • A fully saturated colour is produced by a single wavelength (or narrow band of wavelengths) of light.
  • When any two fully saturated additive primary colours are combined, they produce a secondary colour: yellow, cyan or magenta.
  • Some implementations of RGB colour models can produce millions of colours by varying the intensity 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.

Additive colour and the RGB colour model

About additive colour and the RGB colour model

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 simply by combining the light emitted by arrays of red, green and blue pixels (picture elements) in different proportions.

  • RGB colour is an additive colour model that combines wavelengths of light corresponding with red, green and blue primary colours to produce all other colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours can produce any other colour if mixed in the right proportion.
  • 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 in equal amounts, they produce white.
  • A fully saturated colour is produced by a single wavelength (or narrow band of wavelengths) of light.
  • When any two fully saturated additive primary colours are combined, they produce a secondary colour: yellow, cyan or magenta.
  • Some implementations of RGB colour models can produce millions of colours by varying the intensity 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.

Adobe RGB colour space

About the Adobe RGB colour space
  • The Adobe RGB (1998) colour space is designed to encompass the colours that can be reproduced by CMYK colour printers.
  • 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.

Adobe RGB colour space

The aim of a colour space is to ensure that a colour produced by a colour model is accurately reproduced when displayed on-screen or digitally printed. The Adobe RGB (1998) colour space is an RGB colour space developed by Adobe Systems, Inc.

About the Adobe RGB colour space
  • The Adobe RGB (1998) colour space is designed to encompass the colours that can be reproduced by CMYK colour printers.
  • 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.
About colour spaces
  • 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 digital display, printer or paint mixing machine.
  • When an artist chooses a limited number of tubes of oil paint to add to a palette they are already working within the RYB subtractive colour model and are establishing the colour space within which they plan to work.
  • Digital colour spaces are used to accurately understand the range (gamut) of colours that can be output by digital screens and printers.
  • The Pantone colour collection defines its colour space by:
    • Establishing a set of inter-related colour swatches or samples.
    • Giving each swatch a name or code.
    • Calibrating a paint machine (or other type of equipment) to reproduce the colour of each swatch accurately.
  • When a colour space is to be matched with a specific device such as a projector or printer, a colour profile is used to ensure end-to-end colour management.
  • A colour profile is a program that allows a piece of equipment to know how to handle and process the information it receives so that it can produce the intended colour output.

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:
    • The area inside the arcs of a primary rainbow always appears tonally lighter than the rest of the 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 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 (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:
    • The area inside the arcs of a primary rainbow always appears tonally lighter than the rest of the 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 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.
Related diagrams

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

Amacrine cell functions

About amacrine cell functions

Amacrine cells are known to contribute to narrowly task-specific visual functions such as:

  • Efficient transmission of high fidelity visual information with a good signal-to-noise ratio.
  • Maintaining the circadian rhythm which keeps our lives tuned to the cycles of day and night and helps to govern our lives throughout the year.
  • Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism), so enabling edge detection and contrast enhancement.
  • Motion detection and the ability to distinguish between the movement of things across the field of view and our own eye movements.

Amacrine cells

Amacrine cells are interneurons in the human retina that interact with retinal ganglion cells and/or bipolar cells.

  • Amacrine cells are a type of interneuron within the human retina.
  • Amacrine cells are embedded in the retinal circuitry.
  • Amacrine cells are activated by, and feedback to, bipolar cells. They also have junctions with ganglion cells, as well as with each other.
  • Amacrine cells send complex spatial and temporal information about the visual world to ganglion cells.
  • Amacrine cells are known to add information to the stream of data travelling through bipolar cells and then to control and refine the way ganglion cells (and their subtypes) respond to it.
  • Most amacrine cells don’t have tale-like axons. But whilst they clearly have multiple connections to other neurons around them, research into their precise inputs and outputs is ongoing.
  • Axons are the part of neurons that transmit electrical impulses to other neurons.
  • Neurons are the nerve cells that the human central nervous system is composed of.
About amacrine cell functions

Amacrine cells are known to contribute to narrowly task-specific visual functions such as:

  • Efficient transmission of high fidelity visual information with a good signal-to-noise ratio.
  • Maintaining the circadian rhythm which keeps our lives tuned to the cycles of day and night and helps to govern our lives throughout the year.
  • Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism), so enabling edge detection and contrast enhancement.
  • Motion detection and the ability to distinguish between the movement of things across the field of view and our own eye movements.
About centre-surround antagonism

Centre-surround antagonism refers to the way retinal neurons organize their receptive fields.

  • Centre-surround antagonism refers to the way that light striking the human retina is processed by groups of light-sensitive cone cells.
  • The centre component is primed to measure the sum-total of signals received from a small number of cone cells directly connected to a bipolar cell.
  • The surround component is primed to measure the sum of signals received from a much larger number of cones around the centre point.
  • The two signals are then compared to find the degree to which they disagree.

Amacrine cells

Amacrine cells

Amacrine cells interact with bipolar cells and/or ganglion cells. They are a type of interneuron that monitor and augment the stream of data through bipolar cells and also control and refine the response of ganglion cells and their subtypes.

Amacrine cells are in a central but inaccessible region of the retinal circuitry. Most are without tale-like axons. Whilst they clearly have multiple connections to other neurons around them, their precise inputs and outputs are difficult to trace. They are driven by and send feedback to the bipolar cells but also synapse on ganglion cells, and with each other.

Amacrine cells are known to serve narrowly task-specific visual functions including:

  • Efficient transmission of high-fidelity visual information with a good signal-to-noise ratio.
  • Maintaining the circadian rhythm, so keeping our lives tuned to the cycles of day and night and helping to govern our lives throughout the year.
  • Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism) which enables edge detection and contrast enhancement.
  • Object motion detection which provides an ability to distinguish between the true motion of an object across the field of view and the motion of our eyes.

Centre-surround antagonism refers to the way retinal neurons organize their receptive fields. The centre component is primed to measure the sum-total of signals received from a small number of cones directly connected to a bipolar cell. The surround component is primed to measure the sum of signals received from a much larger number of cones around the centre point. The two signals are then compared to find the degree to which they agree or disagree.

Amplitude

The amplitude of an electromagnetic wave is directly connected with the amount of energy it carries.

In a wave diagram, amplitude is shown as the distance from the centre line (or mid-point) of a wave to the top of a crest or to the bottom of a corresponding trough.

  • As the amplitude of an electromagnetic wave increases so does the overall distance between any peak and the next trough.
  • The greater the amplitude of a wave, the more energy it carries.
  • The quantity of energy carried by an electromagnetic wave is proportional to the amplitude squared.
  • The amplitude of the electric field of an electromagnetic wave is measured in volts per metre and the magnetic field in amperes per metre.
  • Amplitude corresponds indirectly with the perception of the intensity of light and the brightness of colour perceived by an observer but additional factors such as phase and interference must also be taken into account.
About amplitude, brightness, colour brightness and intensity

The terms amplitude, brightness, colour brightness and intensity are easily confused.

Amplitude
Brightness
  • 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 the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.
    • 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 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.
      • The intensity of light 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.

Amplitude, brightness, colour brightness & intensity

About amplitude, brightness, colour brightness and intensity

The terms amplitude, brightness, colour brightness and intensity are easily confused.

Amplitude
  • Amplitude is a feature of electromagnetic waves. Other features include:
    • Centreline
    • Crest
    • Frequency
    • Oscillation
    • Trough
    • Velocity
    • Wavelength
    • Wave-cycle
Brightness
  • 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 the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.
    • 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:
      • The angle at which incidence light approaches a medium, object or surface
      • The composition of incident light in terms of wavelength and frequency
      • The polarization of incident light
    • 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:
      • Adjacent colours: The presence and illumination of adjacent colours can affect both the apparent hue and brightness of a target colour.
      • Attributes of visual perception.
    Intensity
    • 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.
      • The intensity of light 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.