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Translations of Encyclopedia about Physics


The Eye

The eye is one of the most important of the sensory organs and enables a person to orient itself in its surroundings. The construction and functioning of the human eye is similar to a photographic camera. From a physics perspective, the most important parts of the human eye are the iris, the lens and the retina.

The retina is actually an optic organ made up of millions of special cells, around 7 million cones and 125 million stamen, which respond selectively to certain wave lengths of light radiation. Under sufficient lighting, cones enable us to view different colours and to differentiate between sharp contours. Stamen on the other hand allow us to see when it is darker and sees in only grey scales. Stamen and cones capture light and convert it into signals sent to the brain, which processes and organises these signals. On the retina itself, the images viewed actually come out as upside down (the brain then converts the image back to a form we can understand).

The most sensitive place on the retina for viewing colour is the yellow spot, a small area on which is concentrated around 160,000 cones. The location where the optic nerve comes into contact with the eye is without cones or stamen and is called the blind spot.

The iris works similarly to a curtain or shutter and covers the pupil. The iris opens and closes automatically, without our will, and is controlled by two soft muscles which control the amount of light falling on the retina. When there is a lot of light, the pupil opening becomes smaller, becoming larger when there is less light.

Behind the iris is located the eye’s lens, made out of an elastic material. This lens casts a small and opposite image version of the object viewed onto the retina and the soft muscles control the bulge of the lens. Beyond our will, the lens always focuses on an object so that the image of it falling on the retina is sharp, a characteristic referred to as acodomation.

Inaccuracies in this acodomation result from short or far sightedness. With a normal and healthy eye, a clear image (the focal point) is projected on the retina. With a short sighted person however, this focal point falls short of the retina, or behind the retina in the case of a far sighted person, something which can be corrected by wearing properly adjusted correction glasses.

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