It is a complex organ that gathers light from the outside world, organizes it, then sends messages to the brain. The backside of your eye is called the retina, and works sort of like a movie screen. Images are projected on the retina by the lens at the front of your eye. The lens has a very important job because it has to focus light images on the retina. Light images are focused by a process known as refraction.
Refraction is dependent on the speed at which light travels.
Light traveling in outer space is the fastest thing in the world but it can be slowed down by traveling through a medium such as air, water, or even glass. The type of substance it travels through greatly effects its speed. You can easily see how light can be refracted by placing a spoon in a clear glass of water with some of the handle sticking out. When you look at the spoon with its parts in and out of the water, the part in the water will appear magnified and distorted. This is what refraction is.