Iris automatically adjusts the size of Pupil according to the intensity of light.
Hence, the iris controls the amount of light entering the eye by adjusting the size of pupil.
If light is bright, it reduces the size of Pupil,so that less light can enter the eyes
If light is dim, it increases the size of the pupil so that more light can enter our eyes
Adjustment of the size of pupil takes time
If we come from sunlight into a dark room,we are not able to see anything for some time