Physics of Colour

Did you know that a lot of scientific principles are involved in the way we perceive colours? An object looks coloured because of the way it interacts with light. The study of this interaction and the factors determining it are the interests of the physics of colour. This article concentrates on the physics of colour.

What is Colour in Physics?

In physics, colour is associated with electromagnetic radiation of a specific range of wavelengths visible to the human eye. The visible light spectrum is the part of the electromagnetic spectrum that spans wavelengths from 380-750 nanometers and includes all of the rainbow colours such as red, orange, yellow, green, blue, indigo and violet.

The various wavelengths of light are what determine the colours we see. For instance, wavelengths of about 680-740 nanometers appear red, while a wavelength of 550 nanometers appears yellow-green. On the other hand, the colour black is the absence of any colour. Black is the colour that the eyeball perceives when no light strikes it from the location that appears black. To summarize, we can say that the actual colour of an object is defined by its absorptive or reflective characteristics. The figure below shows what happens when white light falls on four different objects, one pure blue, one pure red, and one black and one pure white.

Usually, when you view an object and perceive a distinct colour, you may not essentially perceive a single frequency of light. As an example, consider a shirt that appears yellow to you. In this case, several frequencies of light strike your eyes with different degrees of intensity. However, your eye-brain system translates the frequencies that strike your eye and decodes the shirt as being yellow. The perception of different colours may become apparent to us when we think in terms of primary colours. In the next section, let us go over the primary colours of light.

Primary Colours of Light

When we speak of white light, we primarily associate it with the entire visible light spectrum (VIBGYOR). But did you know that combining the range of frequencies in the visible light spectrum is not the only way of creating white light? White light can also be created by mixing only three distinct light frequencies, given that they are broadly separated on the visible light spectrum. Therefore, any three colours of light that create white light when combined with the right intensity are known as primary colours of light. The most common set of primary colours is red (R), yellow (Y) and blue (B), as they match the sensitivities of the three colour sensing cones of the eye. When the red, blue and yellow light is mixed together with the proper intensity, white (W) light is obtained. This is often represented by the equation below:

R + Y + B = W

Orange (O), Green (G) and Violet (V) are referred to as secondary colours of light since they are produced by the addition of equal intensities of two primary colours of light.

Complementary Colour of Light

Any two colours that produce white light when mixed are known as complementary colours of each. In the traditional RYB color model, the complementary colour pairs are red–green, yellow–purple, and blue–orange. For example, the complementary colour of blue light is orange. This is expected since orange is the combination of red and yellow light, and when red and yellow light are added to blue light, it will produce white light. Thus, blue light and orange light represent a pair of complementary colours of light; they add together to produce white light. Hence, we can generalize that each primary colour of light has a secondary colour of light as its complement.

Colour Vision and Colour Blindness

Colour perception in the human eye is governed by cone cells. Cone cells can identify red light, blue light and yellow light. Each eyeball consists of 7 million cone cells. Based on the relative intensity of the light and the wavelengths that reach the cells, the combination of red, yellow and blue reception enables humans to perceive all the colours in the visible light spectrum.

People considered colourblind have shortfalls in their cone cells. Most colourblind people are unable to see only some colours; being unable to see any colour at all is very rare. Colour blindness affects about 8 per cent of men and 0.5 per cent of women.

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