Colour

Have you ever wondered how we see in colour? Most of you take colour for granted, but none of the Pups, a lot of other animals and even some humans, can't see in colour the way most of you can.

Our ability to see colours depends on the retina - the part at the back of the eye which is a bit like a curved cinema screen. Images that are focused by the cornea and lens land on the retina. The retina has lots of cells in it that are sensitive to light. Some of these are called rods and the others are called cones.

The rods are arranged around the edge of the retina and are sensitive to dim light. Next time you’re outside at night (and it’s not cloudy!), notice how some stars seem to disappear when you look directly at them, but reappear when you look slightly to one side. This is because their dim light can only be picked up by the rods in your eye, which aren’t right in the centre of your retina.

The cones are sensitive to bright light and, importantly, colour, and are positioned right in the centre of your retina. That’s why when you want to see what colour something is, you look directly at it. It’s much more difficult to tell what colour something is if you can only just see it out of the corner of your eye.

You have a lot more rods in your eye than cones. There are about 120 million rods and 6-7 million cones (so you can imagine how tiny they all are!). So you don’t get them mixed up, remember, c is for cones and colour!

Some people’s cones don’t work properly, and some of these people are totally colour-blind, which means they can’t see colour at all. Fortunately, that’s quite rare, but much more common is a form of colour-blindness where greens and reds get mixed up. About seven men in every hundred and four women in every 10,000 have this kind of colour-blindness, but it doesn’t usually cause them any major problems.

Imagine the world with no colour. Traffic lights might be confusing and rainbows wouldn’t be nearly so impressive! This test show how colour-blindness can effect your vision. 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.

What other situations can you think of where colour is important?