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.
This DICTIONARY OF LIGHT, COLOUR & VISION contains a vocabulary of closely interrelated definitions 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.
Other sections
As well as definitions several other sections are available via the DICTIONARY in the MENU (top left of screen). They include:
- Summaries: A section containing short version of definitions.
- About: A section which explains the link between related concepts.
- Rainbows: A section dealing with definitions specific to this topic.
- Vision: A section dealing with definitions specific to this topic.
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.
Show me the DICTIONARY OF LIGHT, COLOUR & VISION
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Absorption of light occurs when the frequency of the wave matches the frequency of electrons orbiting atomic nuclei in a material.
- When light is absorbed by an object or medium, its energy excites electrons, causing them to vibrate more vigorously and collide with other atoms, which in turn produces heat.
- Materials selectively absorb photons whose frequencies match their frequencies.
- Reflected light bounces off a surface at the same wavelength with little or no change in energy.
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 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 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 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.
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.
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.
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 chemiluminescence. Solar 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 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, 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 the 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.
Amacrine cells are interneurons in the human retina within the eyeball that interact with retinal ganglion cells and/or bipolar cells.
- Amacrine cells are an example of neurons which are the nerve cells that comprise the human central nervous system.
- Amacrine cells are activated by and provide feedback to bipolar cells. They also form junctions with ganglion cells and communicate with each other.
- Amacrine cells send complex spatial and temporal information about the visual world to ganglion cells.
