HEALTHY
The cornea,
the external, transparent part of the eye, and tear film refract the light, focusing it at the lens.
The iris
controls the quantity of light that is allowed to enter by regulating pupil size.
The lens
is flexible, and muscles make it contract to focus the light at a sharp point on the retina.
The optic nerve
is a thick bundle of some one million nerve cells, which send information about the visual impression to the centre of vision at the back of the brain.
The retina
includes light-sensitive nerve cells, rods and cones, which send nerve signals to the optic nerve, when they are activated by light.