The iris is the colored part of the eye with a black opening, the pupil, at its centre. The pupil΄s size varies with the brightness of the light. The pupil contracts when exposed to bright light and dilates when it is dark. At the back of the eye, the retina is lined with two types of light-sensitive cells, called cones and rods because of their distinctive shape. The two types of cells do not have the same sensitivity to the light spectrum. When they are excited, a nerve signal is sent to the brain through the optic nerve.