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. These specialized cells are called photoreceptors.
Rods are sensitive to light levels and cones are sensitive to colour.
When the light levels are low, the light focussed on retina needs to be amplified, so that a cat can see in the dark. Behind its retina, a cat has about 15 layers of cells that reflect light. They are called the tapetum lucidum. These act like a mirror, reflecting light back onto the photoreceptor cells in the retina. When light-beams enter the eye, they first reach the rods and cones on the retina.