Dissectible model eye

A model eye and a sectional drawing of the human eye are useful to show students, and links with the biology department can usefully be made when you are teaching how cameras work.

The eye and the camera have a lot, structurally, in common.

• The eye pupil is just a hole which lets the light in.
• The iris is an adjustable diaphragm which controls the amount of light allowed into the eye.
• The retina is a fine network of nerves which are sensitive to light. Most of the adjustment of the eye, between bright sunlight and complete darkness, is done by changes to the sensitivity in the retina so that the range in sensitivity may vary over a million times by storing up light sensitive chemicals for use in low light levels.
• A camera is painted black inside; so is the eye, to lessen trouble due to stray light being reflected.
• Most cameras have a flat film or CCD plate at the back. Some cheap ones hold the film on a curved back to allow for curvature of the image field of their simple lens. The eye has a curved back.

The camera lens must be moved in and out to focus the image of objects at different distances. The eye changes the power of its crystalline lens system by means of ciliary muscles which change the curvature of the lens. The power of an eye differs from one person to another. In round terms, the bending of the cornea is about 40D and the total power may be 60D or more.

The most important thing about the lens is that it can be squeezed to change its power. In an average eye the lens can increase by 4D when it is pulled into greater power to focus something nearby. That change for focusing objects at different distances is called accommodation.

Bear in mind that eyes do not have air in them. The materials inside an eye differ from one another in refractive index. The aqueous humour between the cornea and the lens is salty water that carries chemicals to nourish the cornea. It also presses the cornea outwards to keep it fully rounded. The vitreous humour is a less dense, clear, watery jelly which helps to keep the eyeball fully rounded.

The main refraction of light by the eye is at the front surface, the cornea, hence the success of contact lenses. Since most astigmatism is due to unequal curvature of the cornea, a spherical contact lens can remove astigmatism.

Three parts of the retina are worthy of note;

• There is an interesting patch on the retina where all the nerves of the retina are bundled together into the optic nerve to go to the brain. There are no nerve endings there so it is a blind spot. That gives no trouble in seeing because our eyes scan all the time so you never notice your eye missing anything which falls on the blind spot for an instant. It is easy to show that the blind spot is really there by staring fixedly at X with their right eye, closing the left eye...

• ...while bringing the page nearer. The spot disappears when its image falls on the blind spot. The blind spot is some way from the yellow spot for best seeing(see 3, below.)
• The retina is fed by blood vessels which are just in front of the nerves so that light forming an image goes through them before reaching the nerves. Some people observe a difference in colour due to that red filter when they lie on one side on a sunlit lawn and compare the hues they see with the upper and lower eyes.
• The human eye has a small patch of retina where there are no blood vessels in front. That is the patch used for accurate seeing, as in reading, and is called the fovea or yellow spot. You can learn about the size of it by staring at a book with one eye open and the other closed and estimate the longest word which can be read without moving the book or your eye. However, the eye has a power of 60D or so, the real yellow spot is less than the length of the word. It is at least 20 times smaller and probably less still as it is difficult to keep your eye still when looking at the word. Measurements show it is less than a 1/4 mm wide.

A cheap, fixed-focus camera has such a small aperture that, even if an object is out of focus, the cone of rays from the lens to each image point is so narrow that it makes only a small blur patch if the film catches it too soon or too late. The eye pupil closes down in bright sunlight, thus giving some depth of field. In an emergency, a person who has lost his or her spectacles can read the telephone directory by putting a card with a pinhole in front of the eye and moving closer to the page.