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See the diagram.

The eye is in a socket in the skull and is at a comfortable height in the body, so we can see better.

Human eye is approximately spherical in shape of diameter 2 to 2.5 cm.  It is transparent to light rays.  

The front of the eye contains a cornea, bulged out a little. Then a layer of pupil is behind it.  Behind the pupil we have an eye lens.  Eye lens is a converging (convex) lens.  For a healthy adult the focal length is around 2 cm.  The image of any object seen by the eye is formed on the retina (screen of the eye) on the  spherical surface on the back side.

The nerves and neural receptors behind retina take the visual signals to the brain.  The muscles of the eye change the shape of the front side, so the focal length is adjusted.  The reason is eye has to change focus to see distant object and nearby objects.

There are mainly three types of sight problems, namely, hypermetropia, myopia and astigmatism.  They are corrected by convex , concave and cylindrical lenses.

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