Colour And Vision

Electromagnetic (EM) Spectrum

The Electromagnetic spectrum tells us what wavelengths and frequencies correspond to which types of radiation. The very small fraction of between UV (Ultraviolet) and IR (Infrared) is Electromagnetic (EM) Spectrum called visible light and this is the range of wavelengths which our eyes can see, hence the name visible light. The visible light spectrum stretches from a wavelength of 300 nm to 700 nm and all colours we can see are within this range.

Colour and the eye

In the back of our eyes we two types of cells which receive light, rods and cones. It is cones that interpret colour. Cone cells can also be divided into 3 different types long cones which react to red light, medium cones which react to green light and short cones which react to blue light. However, as you may have realised, we can see more than just red, green and blue. This is due to the fact our three types of cones work together to show us a mixture of colours, for example a wavelength which would show the colour yellow will lead to both the green cones and red cones reacting to a certain degree. This will send a specific impulse to the brain which will perceive the given wavelength as what we call yellow

From any spectrum you will see, there is no purple. We know a mixture of red and blue makes purple. However, wavelength of red and blue in our colour spectrum gives us green, so how do our eyes react to this? When a wavelength from a purple object enters our eye, both of our red and blue cones react, but the actual wavelength received is the same as green. As the green cone isn t reacting our brain `panics`, and results in showing us purple. So what you re seeing isn t purple, you re just not seeing green.

Impossible colours

The scenario described above is known as an Impossible Colour, and here are three main categories that these colours for into, but first we must understand Figure 1. The graph shows the energy expended by each of the cones when a certain wavelength enters the eye. As established, a green wavelength is an average of blue and red, therefore anything the green cone picks up, the blue cone and red cone will also react to, even if minute. This means we will never see pure green, or in other terms, hyperbolic green.

Figure 1: Spectral sensitivities of human cone cells

This leads to our first category of impossible colours, the HYPERBOLIC COLOURS. Hyperbolic colours exist in theory and in practice, but due to our perception of the world caused by how our cones and brain interact, we will never be able to see these hyperbolic colours through our normal vision.

The second types are these are simultaneously dark and impossibly saturated. For example, to see "stygian blue": stare at bright yellow to cause a dark blue afterimage, then on looking at black, the blue is seen as blue against the black, also as dark as the black. The colour is not possible to achieve through normal vision, because the lack of incident light (in the black) prevents saturation of the blue/yellow chromatic signal (the blue appearance). STYGIAN COLOURS

The final types are these mimic the effect of glowing material, even when viewed on a medium such as paper, which can only reflect and not emit its own light. For example, to see "self-luminous red": staring at green causes a red afterimage, then on looking at white, the red is seen against the white and may seem to be brighter than the white.

CONCLUSION

I hope after reading this you have understood the reasoning as to why some colours are impossible in theory yet exist in real life or vice versa due to the way our eyes and brain interact and perceive light, and how we are all colour-blind in front of the reality of what is actually there compared to what our brain makes us see.