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In order to see, your eye must focus light on the retina, convert the light into electrical impulses, and send those impulses to your brain to be interpreted.  

Focusing the light. 

When light bounces off an object and reaches the eye, it must be bent so that its rays arrive at the retina in focus. Four different surfaces bend the light as it enters the eye: the cornea, the aqueous humor, the lens, and the vitreous humor. When all four of these bend the light appropriately, you see a focused image of the object. The eye can focus objects at different distances because the ciliary muscles push and pull to make the lens change shape. When you look at an object that is far away, the ciliary muscles relax and the lens has a flattened shape. When you look at an object that is close by, the ciliary muscles are contracted and the lens is thickened. This is one of the features that makes the eye superior to any man made camera. To adjust a camera lens for the distance of an object, you must move the whole lens forward or back. If our eyes worked the same way, we would need long tubes sticking out of our eyes so the lenses could move back and forth. Instead, our lenses just change shape to adjust for the distance of an object. 

In addition to focusing the light, your eye can control how much light gets in. The colored part of your eye, called the iris, controls the size of the pupil, the opening that lets light through. In dim light, the iris will cause your pupil to expand, allowing as much light as possible into your eye. In bright light, the iris causes the pupil to contract so that less light can enter.

What happens when the focused light reaches your retina? It triggers a complex chemical reaction in the light-sensitive rod and cone cells. Rods contain a chemical called rhodopsin, or "visual purple," and cones contain chemicals called color pigments. These chemicals undergo a transformation that results in electrical impulses being sent to the brain through the optic nerve.

Interpreting in the brain. 

When the electrical impulses arrive in the visual cortex of the brain, the brain analyzes the color and light information from the rods and cones and interprets them as light. The brain flips the image (the light was projected on your retina upside down) and fills in for the blind spot if necessary. All this happens almost instantaneously, allowing you to read a book or enjoy a beautiful sunset. Some of the information from the retina is sent to the visual reflex system in your brain.