Vision is the perception of light

Quite exceptional value in human life is the organ of vision that allows you to clearly and fully aware of all the things that surround the body. Through vision , we get 90 % of all incoming of information into the brain. Not casually so huge role of view in our work.
The eye is often likened to a camera. Indeed, there is considerable similarity in appearance. Eye consists, first, of a lens, i.e. series of refractive lenses that collect light rays at one point and allow you to place an image of great items in small areas of the retina. Secondly, the eyes are equipped actually photosensitive emulsion - special substances chemically modified under the influence of light and thus to send signals to the brain. These substances are placed in a special way arranged the receptors of the retina, called by them the form of rods and cones. Cones are located just in the center of the retina responsible for color vision. Light waves of different frequencies, i.e. different wavelength, different effects on the substance of the cones, and why is the perception of different colors. Sticks are scattered throughout the retina and is sensitive only to the white light, but to a much greater extent than cones to separate colors of the spectrum. Therefore, in the twilight, when the perception of colors already present, we still discern the outlines of objects, but only, so to say, in black and white. They all seem equally gray. Substance, disintegrating in the sticks under the action of light and the sending signals to the brain, is the so-called visual purple, rhodopsin. Part of nature made vitamin A. So-night sight and suffering without this vitamin. Breaking down the light, rhodopsin in the dark restored. The more it has restored, the eye is more sensitive to light. So, after being in the dark for some time, thanks to the recovery of a significant part of rhodopsin begin to recognize objects, previously virtually undetectable. Such devices eyes to the light conditions, also applies to the adaptation phenomena. After an hour of being in the dark adaptation increases the sensitivity of the eye 200 thousand times. But do we think about this wonderful property your eyes! Let's add that an electrical signal, resulting in the decay of rhodopsin in the sticks, United with them nerve cells of the retina reinforce a million times, only then the energy can Give a nerve impulse that rushes to the brain.
If taking a rabbit and, keeping it 3-4 hours in the dark (to restore the whole visual purple), show him for a moment, lighted object, and then, again in the dark, to remove the eye and have done to him alum suspending further disintegration of rhodopsin, the retina to see the image shows the subject. Where had the effect of light and purple collapsed, retin will pale in other places - pink. Clearly, if the rabbit will have time to look at a few items, the experience will not succeed.
Let us now return to the first division eyes - lenses that collects light rays in a narrow beam, with a focus on the retina. The main lens is a lens. When we look at a distant object, which are almost parallel rays, the lens becomes more flat. From the middle of the subject are diverging rays, which have to refract in a greater degree, to give the focus at the same point. Therefore, when looking at a close object, the lens becomes more convex. These changes lens is called accommodation. They're managed by the higher departments of a brain. In some people the lens refracts too much and focus occurs not on the retina, but in front of it. When it comes to nearby objects, which require strong refraction coming from them rays, it does not interfere with vision. Distant same subjects seem vague, for their image on the retina is not in focus. Such people are called short-sighted. They reduce excessive bulge of his lens through biconcave lenses - points.
There also exists another state. The thing is that with age, the lens loses its ability to accommodate, i.e. becomes, if necessary, more convex. For short-sighted, who he is too convex, it doesn't matter: they remain short-sighted life. With normal vision decreases with age the ability to see near small items. In such cases say hyperopia and fix her glasses with a biconvex lens. It is clear that far these people see no better than. in his youth, but, anyway, not much worse. Only in this sense they can be called far-sighted.