The Science of Colours

Vision

The ability to experience the surrounding made a big leap in the evolution of late-stage multicellular organisms. They became responsive to Electro-Magnetic Radiation (EMR) of a certain range. Until then, living organisms had to make contact with everything nearby them for biological needs like food. They are in constant search of food which made them get into trouble like predators and accidents. Vision helped them to be aware of their surroundings, to find food easily, and to stay away from danger. Life obtained many other senses during evolution but most of them are lost, while vision is so important that still, nearly all multicellular organisms carry it.

Why we only see a particular spectrum of light?

Humans only see a particular EMR ranging from 390 nm to 700 nm (nano-meters) while dogs can see from 300 nm to 700 nm. Bees particularly see ultraviolet which helps them to find nectar containing areas in a flower. From all these, it is certain that all the organisms see nearly the same spectrum of light. Sun, our source of energy only produces a spectrum of light in which Infrared Radiation is dominated by more than 50% and visible and UV takes the remaining. By the statistics, organisms should also see IR but in reality, they are not. The reason dates back to the early stages of life forms trying to acquire vision. At that period life still didn’t evolve to live on land. Water has an ability to absorb almost 99% of Infrared Radiation and some part of UV in just a meter depth. These special characteristics of water made the life under it expose only to visible light and some parts of UV, so the organisms developed a response to only these frequencies. Since then, this trait of life is carried to date.

What are colours?

To know this, we have to first understand how we see things. If something is visible, it means light reflected by the object reaches your retina. The retina consists of two major types of cells named rods and cones. Rods detect the intensity of light the retina is receiving and cones detect what frequency of light is received. These two types of cells make the perception of vision. Colours we see are formed due to the stimulation of cone cells. Cone cells are divided into 3 types know as S-cones, M-cones, L-cones. These three types of cones are stimulated to short, medium, and long wavelengths of light. S-cones are stimulated with short wavelengths, M-cones are stimulated with medium wavelengths, L-cones with long wavelengths. When Isaac Newton first experimented with visible light, he found out that the long wavelengths are Reds, medium wavelengths are Greens and short are Blues. Later it was discovered that each of the three cones cells is stimulated by three different ranges of wavelengths. It is not true that L-cones do not detect blues, but it is stimulated more with Reds. Each of the three cone cells is stimulated with all wavelengths but, they are stimulated more with a certain range of frequencies. Every colour you see doesn’t need to have a wavelength, for example Magenta. This colour is produced when the retina receives both Blue and Red wavelengths.

How are different colours formed?

Colours are formed when something gives off light by itself or by reflecting of light. Colours perceived by light emitting objects is such that, if the wavelength of light emitted is between long and medium wavelengths, the M and L cones are activated (Each cone can detect a range of wavelengths). These two stimuli produce a colour resembling that its wavelength is between long and medium wavelengths. This is done for every wavelength of light in the visible spectrum except Magenta. Magenta doesn’t have a wavelength, it is just perceived as colour when retina receives both Red and Blue wavelengths. Colours of objects which reflect light are such that, when they are under a light source, they tend to absorb some wavelengths of the light source and reflect the remaining. Colours are formed by two types of colour groups, namely Additive and Subtractive. Each group contains three main colours.

Additive colours

This group consists of Red, Green and Blue. These three colours add up to form different colours. Red and Green light combine to produce Yellow, where M-cones and L-cones are stimulated. Green and Blue produce Cyan, where M-cones and S-cones are stimulated. Red and Blue produce Magenta, where L and S cones are stimulated. Most of the wavelengths are achieved by varying (Increasing or Decreasing) the stimulation of the cones. Orange is an example of more Red and less Green, that means L-cones are stimulated more and M-cones are stimulated a bit less. White is produced when all three colours are added (all cones are stimulated).

Subtractive colours

This group consists of Cyan, Magenta and Yellow. Each of these colours represents that its inverse colour is absorbed by an object. If an object has any of the above colours, it means the object absorbs the inverse colour of what you see. When two inverse colours are added it gives white. The inverse colours of Cyan, Magenta and Yellow are Red, Green and Blue. Objects having subtractive colour groups cut of specific range of wavelengths by absorbing and reflect back the remaining wavelength. So when you see a lemon (Yellow) in sun light which is white, you are actually receiving wavelengths of both Red and Green because the lemon absorbed the blue light from the received white light. Mixing these colour lead to decrease in range of visible spectrum in reflected light.

Colours in Everyday life

Colours which we see everyday are mostly a mixture of subtractive colours. Every object attains colour by reflecting of some of light emitted by a source (Sun, Light Bulbs, etc). When you take a picture of a beautiful flower and review it in your mobile screen, you are not seeing the same colour as the flower reflects. It’s actually your mobile screen which is making an illusion of colour and made you think that it’s the same colour. Electronic displays produce images by emitting light with pixels. Displays nowadays contain millions of these pixels. But each pixel only emits RGB wavelengths. RGB is the additive group and they add up to form a spectrum of different colours.

Colour is an illusion created by our brain to distinguish between different wavelengths of visible spectrum.