Comparatively small in size, the eye is a very complex organ. How the vision developed in the evolution and how it works?
To understand the working of eye in simpler terms, one can compare it with a camera. Both the eye and the camera can be considered as the diaphragm. In the 'sensitive film' is the retina which capture the images that fall on it. The process starts with the ray of a light.
When this ray of light reaches the eye it bends by the system of the lenses (intraocular lences). And therefore the ray of light finally falls on the retina after crossing the lences. The image which is formed through that is upside-down (inverted) and is also reduced in size. The image only gets its meaning when it (information) reaches the visual centre of the brain via the optic nerve (cranial nerve). Important thing to remember is that here the images of both eyes are combined and then interpretation takes place. Problem at any of the area or pathway disrupts this precess and various disorders come into the picture.
Dear Monu. You can visit this link for a beautiful explanation as the answer to your question.http://www.pasadenaeye.com/faq/faq15/faq15_text.html
one more thing we should know is about the perception of vision, which includes motion, depth and color. these all are a bit complex but very interesting
First evidence of Vision comes into the play with the presence of photoreceptors in lower invertebrates. The first organisms to exhibit eye spots are helminths which belong to quitelower order of invertebrates and this keeps on getting better in senstivity from there onwards, arthopods have complex structures called compoud eyes. even in vertebates the Photoreceptors are continously evolving like amphibians, frogs and teleosts have absolute spherical lenses which are mobile and birds and human s have biconvex lenses which are fixed but the rear half is adjustable for the shaper focussing. You can imagine the resolution power of your eye which has the capability of clearly focussing the objects as close as few centimeters to stars which are millions of light years away.
Compared to a camera, vision is extraordinarily complex. Before a signal ever leaves the retina, it is integrated, processed, and relayed by 5 different cell types, each of which has multiple subtypes. Most people do not realize that the eye does not merely recieve light and transmit the signal to the brain, but pre-processes the signal in many ways. One of the most prominent examples is the retina's ability to tell signals apart and strenghen the signals at the borders of objects, so that we are more responsive to change.
Explaining how it works would take a book, and there are a fair bit of them, actually. If you want to read more on it I'd recommend "The Retina" by John Dowling. A little old, but very good.