In very basic terms, the eye has an opening for light to enter, a lens system to correctly focus the light and a screen upon which the image is projected. 

Rays of light coming from an object enter the eye through a multi-layered transparent dome at the front of the eye. This is the cornea, which is the first lens in the system. It begins the bending or refracting of the light rays back toward a point of focus on the retina. The retina is the back of the eye upon which the image is projected.

After passing through the cornea, the light rays travel through the pupil, which is the round black circle within the coloured part or iris of the eye. The iris acts like a circular window shade, making the pupil larger or smaller as the need for light changes. Right behind the pupil the light rays pass through the crystalline lens of the eye. It is a flexible lens and its shape can be altered by a circular muscle at its circumference. Its function is to make the small final adjustments in focusing. 

Behind the pupil the now correctly focused rays of light pass through a gel-like substance called the vitreous and arrive at the retina. The retina is a transparent membrane containing a million light sensitive receptors. Those receptors send their signal to the brain which analyses the signals and “we see”.

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