Can anyone describe how the eye focuses light on the retina?
Light is focused onto the cells of the retina by the lens. The lens of the eye can have its shape manipulated by ciliary muscles that can pull on the lens and change its shape. This is important so that near and far objects can be focused upon. The iris also plays a role in regulating how much light can enter the eye to be focused upon the retina. This can be demonstrated using the accommodation reflex. Ask a friend to look over your shoulder into the distance, then tell them to focus on an object close to their nose (your finger for example). You should see that their eyes converge (i.e. they go cross eyed) and their pupils constrict. This is because the lens is having to focus very divergent light rays, so less light is allowed into the eye by the the iris..
