All of us at some time or the other have seen or used lenses. Lenses naturally occur in eyes and are a good example. Lenses are used in cameras, microscopes, binoculars and many other optical equipment.
Lenses are optical elements used to focus or defocus images.
They are generally made of high quality glass, but these days, high quality plastics or polymers are also used. The surface of a lens can be convex, concave, plane or a combination of these. A lens is not coated as a mirror, but because of the way the surface is curved, they can make light rays passing through them diverge or converge. They refract light rays and not reflect them. A convex lens focuses light rays coming from infinite distance at its focal point.