A lens is a shaped piece of transparent glass or plastic that refracts light. When light is refracted, it changes direction due to the change in density as it moves from air into glass or plastic. Lenses are used in cameras, telescopes, binoculars, microscopes and corrective glasses. A lens can be convex or concave.
In a ray diagram, a convex lens is drawn as a vertical line with outward facing arrowheads to indicate the shape of the lens. The distance from the lens to the principal focus is called the focal length.
A concave lens is thinner in the middle than it is at the edges. This causes parallel rays to diverge. They separate, but appear to come from a principle focus on the other side of the lens.
In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.