When the parts and process of vision work correctly, you see sharp, colored entities out in the world. When things go wrong, the need for a contact lense or glasses arises.
The human visual system is both simple and complex. Complex, because there are so many parts that have to work together to make sight possible. Simple, because those parts and the process can really be explained without in-depth knowledge of anatomy and biology.
An ordinary glass lens, such as a magnifying glass, focuses the light that moves through it by bending the rays toward a focal point. That's the bright spot you see when you hold one over a newspaper, for example. Something similar happens when light enters the eye through an opening called the pupil, behind a clear protective portion called the cornea.
The pupil is surrounded by the iris, the colored portion of the eye. The pupil can narrow or widen to regulate the amount of light admitted, from as small as about 2mm to as much as approximately 8mm. The iris contains the muscles that change the diameter of the opening. Just behind the pupil is the lens, the part that focuses the light rays similar to the magnifying glass.
But there's an important difference between a magnifying glass and the eye. The magnifying glass's focal point is fixed. The eye, by contrast, contains muscles that help shape the lens and that shaping process performs a number of important tasks. Most importantly, it makes it possible for the focal point to change.
A camera lens is a good analogy. Because the camera lens contains multiple pieces of glass that can be moved by turning the barrel, objects at different distances can be brought into focus on the film. The muscles attached to the lens perform a similar function in order to focus light onto the retina, which is like the film.
Focused light rays strike the cells of the retina, the cones and rods. The names come from the shapes of the cells. The cones - 7 million cells in the central part of the retina, called the macula - are responsible for sharp, detailed vision and color vision. The approximately 100 million rods around them help us see in dim light and provide peripheral vision.
When those focused light rays hit the cells chemical reactions occur that stimulate the optic nerve attached to the back of the retina to produce electrical impulses. Those impulses are then transmitted to the visual cortex, the part of the brain that organizes them. Many parts of the brain cooperate to cause the mind to conclude: 'Ah, a car.' But the visual cortex does the initial heavy lifting.