The eye

The eye is one of the most incredible parts of your body. When your eyes are working well, they can see the hairs on a spider’s legs or galaxies that are billions of miles away.

The eye is about the size of a squash ball, but with a bulge at the front. Look at a friend’s eyes - the bit you can see is where the bulge is. You’ll also see a black spot in the middle, called the pupil. This is where light gets inside the eye. Around the pupil is a coloured ring, called the iris. This might be brown or green or blue, or even a funny browny-greeny-grey! Everyone’s iris is different, which means that no-one has eyes just like yours - you’re unique! The iris is actually lots of tiny muscles. As these muscles contract or relax - become tighter or looser - your pupil gets bigger or smaller, to let in more or less light.

To understand how this works, imagine a group of friends arranged in a circle, all pulling (gently!) with one finger on a rubber band. As they all pull, the hole in the middle of the rubber band gets bigger. In the eye, the hole in the rubber band is the pupil and your friends’ fingers are the tiny muscles in the iris.

The pupil gets bigger when it needs to let more light into the eye, for example when it’s dark. It gets smaller when your eye needs less light, for example in bright sunshine. To see this work, get a friend to cover their eyes for 30 seconds, then open them and look directly at you. When they do, you’ll see their pupils get smaller as they adjust to let in less light. It doesn’t take long though, so you’ll have to be quick!

Once light has gone through the pupil, it is focused to give a sharper picture, first by the cornea, then more precisely by the lens. It then reaches the part of the eye at the back called the retina. The retina then changes this light into a message that it sends down the optic nerve to the brain.