Colour

We humans are rare, although not unique, in our ability to see in colour. The reason for this ability lies inside the eye.

Our eyes contain many millions of structures called rods and cones. The 120 million rods are more than a thousand times as sensitive as the cones and are particularly good at sensing low-level light, but not colour. The 6-7 million cones, on the other hand, are what give us the ability to see colour (so you don’t forget which is which, c is for cones and colour!). Each cone contains one of three pigments, making them sensitive either to blue, green or red.

The cones are mostly situated in the centre of the retina, with the rods mostly around the edge. This is why we look directly at something to be certain of its colour, and why it is harder to ascertain the colour of something out of the corner of your eye.

Colour-blindness

Many people think of colour-blindness as an inability to see any colour at all. This condition, known as typical colour-blindness or achromatism, does exist but it is very rare indeed. The more common type of colour-blindness, atypical colour-blindness or colour vision deficiency, causes difficulties in differentiating between certain colours. In the most common kind, for instance, reds and greens are often confused.

This test shows that individuals with normal colour vision will see a 5 revealed in the dot pattern. An individual with Red/Green (the most common) colour blindness will see a 2 revealed in the dots.

Someone with this kind of colour-blindness has fewer of a particular kind of cone (e.g. red-sensitive) than is usual. Men are much more likely to have this form of colour-blindness than women - around 7 per cent of men have it, as opposed to about 0.04 per cent of women - but it rarely causes them any major problems. In fact, many people don’t even realise they are colour-blind.