It changes the size of the pupil and allows different amounts of light to enter the eye.
When people ask you what colour your eyes are, they mean what is the colour of your iris.
The majority of the outer surface of everyone’s eye is white – this is the sclera.
What do eyes and bums have in common?
Did you know that the iris is far more than just the coloured part? Its actually a sphincter muscle – that’s a muscle that tightens like a drawstring bag.
The most well known sphincter muscle is in your bottom – it’s the one that keeps the poo in!
But in your eye it keeps light out by tightening up when it’s too bright – that’s why your pupil looks like it is getting smaller in bright light!
Try looking in a mirror and changing the light near your eye with a torch.