When the light is intense, the ciliray muscles contract and the pupil becomes small. When the source of light is dim, the ciliary muscles relax and the size of the pupil is expanded to admit more light in the eye. The retina is like a screen on which the light is focused and 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.
The cones secrete the pigment iodopsin and are most effective in bright light. The rods, which secrete a substance called visual purple, or rhodopsin, provide vision in dim light or semidarkness. Thus cones need high light levels to get activated and rods need low light levels to be activated. Cones are more near the centre of the retina. Rods are more near the periphery of the retina.