When it is dark, the iris opens to let more light come in. The transparent lens, about the size of a pea, is located behind the iris. It changes shape so it can focus light rays on the retina at the back of the eye. The cornea covers and protects the lens. It also aids in focusing light.
Muscles control the shape of the lense to allow it to focus on objects near and far away.
Most of the space in eye contains vitreous humor , a clear jelly-like substance that gives the eye its shape. The retina is lined with light-sensitive rod and cone cells. Rod cells respond only to black and white and need only a little light. Cones let us see in color when there is bright light. When an image reaches the retina, it is upside down.