Concave lenses are used for correcting myopia or short-sightedness. Convex lenses are used for focusing light rays to make items appear larger and clearer, such as with magnifying glasses.
A concave lens contains at least one inwardly-curved surface, and it is also referred to as a divergent lens. When light rays enter a concave lens, it diverts, or spreads, the rays around because of its thinner center, causing the objects to appear smaller or farther away. A car's side mirror is one example of a concave lens, and it often comes with the warning "objects in mirror are closer than they appear."
A convex lens, also known as a converging lens, is identified by its thicker middle and thin outer edges. It consolidates the rays of light that travel through it, as opposed to diverting and spreading them the way a concave lens does. When light passes through the lens, it converges into a single point of focus called the principal focus. Camera lenses are convex, as they focus all rays of light onto the subject being photographed to make that subject appear clearer. A magnifying glass is another example of a convex lens; once the lens is placed close enough to the object, it will produce an upright, virtual image of the object.