The eye

The eye is a sense organ containing receptors sensitive to light intensity and colour. Seven structures must be identifiable on a diagram, with their functions.

Accommodation is the process of changing the shape of the lens to focus on near or distant objects. Learn both sequences as three steps each, in order.

To focus on a near object: the ciliary muscles contract, the suspensory ligaments loosen, and the lens is then thicker and refracts light rays strongly.

To focus on a distant object: the ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens is then pulled thin and only slightly refracts light rays.

Adaptation to dim light is a different mechanism, done by the iris. In dim light the pupil widens to let more light in; in bright light it narrows to protect the retina. The iris muscles do that automatically — it is a reflex.

Two common defects are myopia (short sightedness) and hyperopia (long sightedness), in which rays of light do not focus on the retina. In myopia the rays focus in front of the retina, so distant objects are blurred; in hyperopia they would focus behind it, so near objects are blurred.

Generally these defects are treated with spectacle lenses which refract the light rays so that they do focus on the retina — a concave lens for myopia, a convex lens for hyperopia. New technologies include hard and soft contact lenses, laser surgery to change the shape of the cornea, and a replacement lens in the eye.

In a ray diagram, find where the rays cross: in front of the retina is myopia, behind it is hyperopia, and the corrected diagram should show them meeting exactly on the retina.

Reading a ray diagram is a skill in itself, and the specification asks for it directly. Work through it in the same order every time.

First, find where the rays cross. In myopia they cross in front of the retina and are already spreading apart again by the time they reach it, so the image is blurred — the eye refracts the light too strongly. In hyperopia they have not crossed by the time they reach the retina and would only meet somewhere behind it, so the eye refracts too weakly.

Then work out what the lens has to do. Myopia needs the rays spread apart before they enter, so that the eye's own focusing brings them together later — that is a concave (diverging) lens. Hyperopia needs them brought together sooner — a convex (converging) lens. In a corrected diagram the rays should meet exactly on the retina, and if they do not, the diagram is showing you the uncorrected case.

The retina carries two kinds of light receptor, and each is adapted to a different job.

Rod cells are sensitive to low light intensity, so they are the cells you see with at night. They cannot distinguish colour, which is why colours drain out of everything in the dark. They are spread across the whole retina.

Cone cells need brighter light to respond, and they detect colour. There are three types, sensitive to red, green and blue light; the brain works out a colour from how strongly each type responds. Cones are concentrated at the fovea, the part of the retina directly behind the lens, which is why detail is sharpest when you look straight at something.

Colour blindness follows from that. If one type of cone cell is missing or does not work, the brain receives no signal from it and cannot separate the colours it distinguished. The common form is red–green colour blindness, in which red and green look alike.

A cataract is a different kind of fault: the lens becomes cloudy rather than clear. Light is scattered as it passes through instead of being focused, so vision is blurred, colours look faded, and bright lights produce glare. It is corrected by surgery — the cloudy lens is removed and an artificial one is put in its place.