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1.5

Colour and light

What is Colour?

Colour is a property of the way an object reflects or emits light combined with the way this light stimulates the eye to provoke different sensations.

In order for a human to see colour the necessary components are:

  • A coloured object

  • Full spectrum light

  • The eye to collect reflected light

  • The brain to process the information from the eye

Light itself has colour and the colour sensation received in your eye varies with the wavelength of the light. Different colours of light have different wavelengths, so a particular shade of red could equally be described by its wavelength or by the frequency of its wavelength: the wavelength that produces this colour.

The visible spectrum with wavelength information

So, a light wave with the wavelength 415 nanometres can only ever be a specific shade of violet, a light wave with the wavelength 525 nanometres can only ever be a specific shade of green. We have names for the frequencies of light waves – we call them red, or yellow, or turquoise, etc.

The human eye is capable of perceiving a wide range of colours and this is called the visible spectrum. A typical human eye will respond to wavelengths from about 390 to 700 nanometres; this spectrum extends from violet (from 380 nanometres wavelength, and the shortest wavelength light we can see) to red (to 750 nanometres wavelength, and the longest wavelength light we can see). All other visible colours fall between these two extremes of the spectrum.

White light split through a prism By D-Kuru

This is the spectrum of light first identified in a paper written by Isaac Newton in 1672 recording the outcome of his experiments separating white light into its component colours by refracting it through a prism.

How do we see colour?

In sunlight, which emits all colours of light, a ripe tomato appears to be red. This is because the surface of the tomato absorbs all colours of light except for red light. This red light is reflected off the surface of the tomato allowing it to be received in your eye, and triggering the parts of your eye that respond to red light. Other colours of light, or light waves absorbed by the tomato are converted into heat.

White objects reflect all colours of light: no light is absorbed and therefore no heat is generated as a result of light absorption. Conversely black objects absorb light of all frequencies; this is why black clothing will be warmer than white clothing on a sunny day.

Light entering your eye is received at the back of the eyeball on the retina. You have two different types of light-detecting cells in your eye: rods and cones. In low light conditions the rods are responsible for providing you with vision; however, you have only one type of rod cell and it is not sensitive to colour, hence the grey tones of your night vision. In the dark you can’t see the difference between a red or blue pencil.

In good light the cone cells in your retina take over. You have three different types of cone cell which roughly correspond to the colours red, blue and green. When you see a colour, each type of cone cell sends its own distinct message to your brain. Other colours (i.e. not red, blue or green) are communicated to your brain because they stimulate a combination of cone cells simultaneously.

Mixing colours and coloured light

Given what we learn about mixing colour when playing with paint at school, the colours that result from mixing different coloured light can be surprising.

Newton split white light and released the colours of the visible spectrum. By re-combining this spectrum and mixing it back together, once again white light is produced.

Therefore, mixing red light, green light and blue lights results in the creation of white light.

This system of mixing colour is called the Additive System, whereas the system of mixing paint colours we learned in childhood is called the Subtractive System.

Each of these two systems has a different set of primary and secondary colours.

The Subtractive System

In the Subtractive System – the ‘paint’ system – the primary colours are red, blue and yellow, and these combine to make the familiar secondary colours of purple, green and orange.

The Additive System

The primary colours of the Additive System – the system of mixing light – are red, green and blue, and the secondary colours are cyan, magenta and yellow.

The Additive System – the ‘light’ system – that allows the eye to perceive any colour when presented with the correct blend of red, blue and green light underlies everyday technology. For example, it is likely that you are viewing this material on a computer or smartphone screen. Each pixel of this screen contains three small light-emitting dots which produce red, green and blue light respectively, and which can be varied in intensity in order to communicate the full visible spectrum of colour to your eye.

If you can, we recommend you view these excellent animations by Colm Kelleher, in association with TedEd, which illustrate the concepts contained in this article:

What is Color?

How We See Color?

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This article is from the free online course:

The Power of Colour

KLC School of Design

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