As a beam of light passes through a lens, one of three things may occur, depending on the shape of the lens. The light either passes straight through the lens, is bent away from the center of the lens, or is bent towards the center of the lens.
When the lens has absolutely no curve or if the ray passes directly through the center of a lens, all parts of the light beam pass through an equal amount of glass and come out in the same direction that they entered the lens. When a lens is convex or concave, some parts of the light beam travel through more glass than others, and the direction of the beam changes as it leaves the lens on the opposite side.
A concave lens is a lens where the center of the lens is more skinny or hollow than the outer edges. When a ray of light enters, the outside edges of the beam travel through more glass than the inside edges. This causes the ray to exit the lens at an angle away from the center line of the lens. The opposite phenomenon occurs when a ray of light passes through a convex lens - a lens that is fatter in the middle than at the edges. The light towards the center of the lens is slowed down as it passes through more glass, and the ray of light is directed towards the center line of the lens as it exits.