Imagine the eyeball as a small, hollow ball, measuring approximately 2cm from front to back. The only way that light can enter the eye is through the pupil, a small circular opening in the centre of the iris at the front of the eye.
When you look at someone’s eye, the pupil is the small black disc at the centre of the coloured part of the eye, the iris. Tiny muscles in the iris will contract and relax to allow the pupil to change in size and therefore control how much light can enter the eye. On a bright, sunny day you will notice that the pupil is quite small, whilst at night the eye needs as much light as it can get and therefore the pupil is large.
In older people the pupil naturally decreases in size and shows less variation in size.
The inner lining of the tough wall of the eye is the retina, a very thin, light-sensitive membrane. The main cavity inside the eye is filled with a clear jelly-like substance called the vitreous, which for much of your life is pushed up against the retina.
As light rays enter the eye, they must be focussed to form an image on the retina. The focussing is done by the cornea and the lens. The lens can change its focussing power by changing shape. This is controlled by a small ring of muscle around the lens. Unfortunately, as we go through life the lens becomes less flexible and so cannot alter its focus. This is why we all develop problems with focussing on objects at different distances and why so many of us require reading glasses in later life.
Six tiny muscles around each eye control the position of the eye and ensure that when you look directly at an object, the image of that object falls onto the special part of the central retina called the macula. Light falling onto the retina stimulates millions of tiny nerve cells. All of these tiny retinal nerves come together at the back of the eye to form the optic nerve, which carries the image information along to the visual cortex at the back of the brain, allowing you to ‘see’ the image.