The size of the pupil plays a big role in admitting light into the eye. When there is more light, the pupils contact. When the light is dim, the pupils dilate to admit more light into the eye. Ciliary muscles in the eye control this mechanism, both in human eye as well as in animal eyes. In cats, the size of the pupil is controlled by two, shutter-like ciliary muscles.
Cats have a characteristic slit-like pupil in bright light conditions.
This mechanism admits as less light as possible in the cat s eye during daytime. When light level becomes dim, the ciliary muscles relax so that pupils expand and more light enters the eye. Light rays are focussed on the retina. The retina acts like a screen on which an image is formed. Retina contains cells that respond to light.