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The Eye


The Eye

diagram of the eye

Above is a diagram of the eye, it shows all of the major components. Now, here is some more information about the function of some of the individual parts labeled.

Choroid This is a layer in the eye's membrane that contains the blood vessels. It is also pigmented a dark colour so that there is as little light interference in the eye as possible. The layer above this is the sclera which is an outside layer protecting the eye.

The iris consists of muscles that act to adjust the amount of light entering the eye by the size of the pupil.

Cornea. This is a transparent part of the sclera where light enters, it is curved to help focusing by refraction.

Lens. This is a biconvex disc, so that light converges on to the retina. It's shape can be adjusted by the cilliary muscle to allow focusing on near and far objects.

Retina, this is a light sensitive area at the back of the eye that detects light and sends signals to the brain.

Focusing Images

The iris controls the amount of light that enters the eye by contraction and relaxation of the radial and circular muscles in the iris (see here). This increases the size of the pupil so more light enters when it is dark and the reverse in light conditions.

Another way the eye must adjust the light is by accommodation, or focusing; it does this by refraction. Every part of the eye refracts (or bends) light by different amounts. Most refraction occurs in the cornea, because it is curved. However this only every by the same amount, therefore we adjust the shape of the lens to vary the refractive index to focus light on the retina.

The table below outlines the eye's responses to focusing on different objects.

Distant Objects
(less refraction)
Near Objects
(more refraction)
The ciliary muscles relax, this causes the suspensory ligaments to be pulled tense. This stretches the lens and makes it longer and thinner or less convex. This causes less refraction. The ciliary muscles contract, causing the suspensory ligaments to slacken. This makes the lens become shorter and fatter and less refraction occurs.

The light is focused on the retina

Rods and Cones

In the retina are cells responsible for detecting light, and sending this information to the brain. There are two types of cell, the rod and cone. Below is the structure of a rod cell, however the cone cell has the same features labeled, but is differently shaped.

diagram of a rod cell

Rod cells are responsible for detecting light/dark. They contain a pigment called rhodopsin. When light shines on this pigment, it is broken into the two proteins: retinal and opsin in a process called bleaching, this stimulates an action potential that is detected in the brain.

However, the rhodopsin is very sensitive to light, and is therefore best in dim conditions; since in brighter conditions it is broken down faster than it is reformed. This is why, in dim conditions we will see mainly in black and white.

The cone cell has a different pigment called iodopsin. There are three different types of this pigment: each sensitive to either red, blue or green wavelengths of light. Therefore we have red, green and blue cones. It is possible to see different colours by the stimulating of different combinations of iodopsins. For example, orange light is a result of red and green cones being stimulated.

the colour wheel of trichromatic vision

The table below outlines the differences between rods and cones in terms of their sensitivity and visual acuity, which is the degree of detail it can see. So a high visual acuity means that one can see finer details.

RodsCones
Are spread evenly across the retina but there are none in the fovea.

Rod cells are sensitive to low light intensities, so are made best use of at night.

They have a low visual acuity because several rod cells share a connection to the optic nerve. But this also improves the eye's ability to detect small amounts of light.
There is a higher concentration of cone cells in the fovea.

They are more sensitve to high light intensities and therefore colour can not be seen very easily when it is dark.

Cones have a high visual acuity because each cone cell has a single connection to the optic nerve, so the cones are better able to tell that two stimuli are seperate.


Updated: 09 May 2012