The human eye is a complex system whose main objective is the most accurate perception, primary processing and transmission of the information contained in electromagnetic radiation of visible light.
All parts of the eye, as well as their cells are the fullest implementation of this goal.
Eye is a complex optical system. Light rays reflect from the surrounding objects and get into the eye through the cornea. The cornea in an optical sense - is a strong converging lens which focuses the diverging in different directions light rays. Moreover, the optical power of the cornea does not change and always gives a constant degree of refraction. The sclera is an opaque outer layer of the eye, so it can't participate in the conducting of light inside the eye.
Refracted from the anterior and posterior surfaces of the cornea, the light rays pass unimpeded through the clear fluid that fills the anterior chamber up to the iris. Rays which are more peripherally, delayed by a pigment layer of the iris. So the pupil not only controls the amount of light flux on the retina, which is important for adaptation to different levels of light, but also eliminates lateral, random, causing distortion rays. The light is refracted by the lens, as well as the cornea.
Its principal difference is that in people under 40 years old lens can change its optical power - a phenomenon called accommodation. Such a way the lens produces more accurate focusing. Behind the lens is located vitreous, which extends up to the retina and fills a large volume of the eyeball.
Rays of light focused by the optical system of the eye fall ultimately on the retina. The retina is a kind of spherical screen onto which projected the world around us. From school physics course we know that converging lens gives an inverted image of the object. Cornea and lens - are two converging lens and the image projected on the retina, is also inverted. Other words, the sky is projected on the lower half of the retina, the sea - at the top, and the ship, we are looking at is displayed on the macula.
Macula, the central part of the retina responsible for high acuity vision. Other parts of the retina does not allow us to read or enjoy working on the computer. Only in the macula are all conditions for perception small details of objects.