Sci Pry

Five Stupid Beliefs About the Eyes

Glasses and crossing the eyes are not harmful

By Stefan Anitei, Science Editor

12th of July 2007, 17:11 GMT

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The human eye is one of nature's wonders. An adult eye has about 24 mm (1 inch) in diameter and about 12 million photoreceptors (light sensitive cells) and six muscles that move the ocular sphere with such a precision that it allows the eye to follow moving objects. Like a camera, the eye has a diaphragm (called iris), whose opening degree adapts to the brightness of the environment.

The light rays that cross the opening of the iris, the pupil, have been previously

concentrated by the powerful action (till 45 dioptres) of the cornea. The crystalline lens is charged with the focusing of the light and its refraction index (between 19 and 38 dioptres) is adjusted by the ciliary muscles. After crossing the vitreous humour, the light reaches the retina, a membrane possessing 120 photosensitive rods and 7 million color photosensitive cones. On the retina an upside down image is formed, a photo that is reverted by the brain.

The rods and the cones turn the light into electric signals which are transmitted through the ganglionic cells to the optic nerve and from there they reach the brain.

There are many urban legends about the eye and the eye's functioning, many without any glimpse of scientific support.

1."The glasses increase the myopia and the hypermetropia because the eyes accustom the them."

Myopia (distant range vision impairment) and hypermetropia (short range vision impairment) are determined by the eye's form (especially the form of the cornea) and this form is not affected by the use of the glasses.

2."Reading in dim light harm the eyes".

A poor illumination can cause headaches and harm the already weak eyes, but in fact strengthens the healthy eyes.

3."Crossing the eyes produce strabismus".

The incorrect eyes' posture is the result of impairments during the development. Crossing the eyes for amusement is not dangerous.

4."The vein of the iris can show a person's health".

Similarly to the fingerprints, the veins of the iris do not ever change and thus cannot allow to find out the health state of the owner.

5."The blue-eyed ones see better than the brown-eyed ones".

The color of the iris has no influence on the visual sharpness of a person.

TAGS:

eye | vision | iris

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