The human eye is an organ which reacts to light for various purposes. As a conscious sense organ, the mammalian eye permits vision. Cone and Rod cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors.
In common with the eyes of other mammals, the human eye''s non-image-forming photosensitive ganglion cells in the retina receive the light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin  and entrainment of the body clock.

Working of eye
The light coming from an object enters the eye through pupil and cornea. The eye lens converges these light rays to form a inverted, real and diminished image on the retina. The light sensitive cells of the retina gets activated with the incidence of light and make electric signals. These electric signals are sent to the brain by the optic nerves and the brain interprets the electrical signals in such away that we see an image which is erect and of the similar size as the object. Before we go into the defects of vision let us be familiar with the terminology used by the ophthalmologists like least distance of distinct vision, far point and power of accommodation of the eye.