We can think of the eye as a kind of camera. The iris behind the cornea is light-sensitive and corresponds to the aperture of the camera. This can be set near or far, depending on the amount of light falling on it. The light rays penetrate the lens of the eye, and fall on the retina. Millions of light-sensitive cells convert the light into signals sent to the brain via the optic nerve. These are put together to make images, and are interpreted by the brain. There are two types of cells in the retina: the light-sensitive rods that help us see in the dark and the less sensitive cones that help us see colours.