Basic functions of the parts of the eye

The cornea, the transparent area at the front of the eye, allows light to pass through the eye, with the size of the pupil controlling the amount of light by the muscle in the iris contracting or dilating.

The light is refracted by its passage through the cornea and, with the help of the lens, provides a sharp image to the retina by focusing this refracted light. The retina is situates at the rear of the eye and acts much like a film in a camera. Of the total refraction and focusing, about 70% take place in the cornea with the remaining 30%, the focus adjustment, occurring in the lens.

The image on the retina is inverted and reversed.

The retina consists of light sensitive cells known as rods and cones. These convert light into nerve impulses, which pass along the optic nerve into the brain, where they are converted to images. There are no light receptors at the point on the retina where the optic nerve is connected, leaving a ‘blind spot’. This occurs normally in all eyes, although blind spots can also be associated with eye disease.

The rods and cones perform two different functions; the rods are sensitive to low light levels, but not colour, while the cones are sensitive to colour.

The eye adapts to different light intensities in two distinct ways. The iris, the coloured part of the eye, can change diameter, controlling the amount of light that is allowed to pass into the eye through the pupil. In dim light the diameter is very large, while as light intensity increases, this diameter reduces. This can limit the amount of light reaching the retina by as much as 80%. The rods taking over the sensing from the cones further refine this. The fovea is the central point of the retina and consists entirely of cones. The rods increase in number towards the edge of the retina where there are no cones.

As there are only cones in the Center of the retina which are not very good in very low light, it is sometimes better to look 10 degrees either side of an object, thus using more of the rods in the retina, enabling you to see an object more clearly in the dark!

In addition to the iris changing it’s size to allow for different light levels, further changes take place by chemical changes in the retina. These changes take place fairly slowly, but are more effective and can cope with changes in the intensity of light in the ratio of 1:150,000.

Although the eye is very capable of adapting itself to compensate for low or high levels of light, it is still possible for too much light to reach the retina and cause damage. Which is why we should never look directly at bright sources of light, such as the sun.

Most of the retina is used to detect movement and to maintain spatial orientation. It can only resolve fine detail in the central area known as the fovea. When it is necessary to view an object in fine detail, the position of the head and the eye must be adjusted to bring the object into focus on the fovea. This is the area of greatest visual acuity, which is defined as the ability of the eye to resolve fine detail. Only at the fovea will the visual acuity be 6/6 or 20/20. This visual acuity ‘rating’ is what would be found in someone with normal vision. When the first number is larger than the second number, acuity is better than normal, and when the second number is higher than the first, the acuity is not so good.

Visual acuity is a measure of the smallest letter a person can read on a chart at 20 feet, compared to the distance at which a ‘normal’ person could read that letter. So 20/20 vision is normal and 20/60 is the acuity rating of someone who can read at 20 feet a letter that a ‘normal’ person can read at 60 feet.

Visual acuity drops off rapidly. The fovea covers just one degree of vision and outside a ten degree cone of vision, concentric to the foveal cone, only about one-tenth can be seen of what can be seen within the foveal field!

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