The iris controls the size of the pupil in the human eye.
The iris is the coloured ring of tissue surrounding the pupil, which is the opening through which light enters the eye. Circular and radial muscles in the iris adjust the pupil’s diameter. In bright conditions, the pupil generally becomes smaller; in dim conditions, it generally becomes larger, allowing more light to reach the retina.
The iris lies between the cornea and the lens and divides the front part of the eye into chambers containing aqueous humour. Its colour depends largely on the amount and distribution of melanin and on how light scatters through the tissue. The retina, by contrast, is the light-sensitive layer at the back of the eye. The lens focuses incoming light, while the cornea provides much of the eye’s refractive power.