A lens is a transparent piece of glass or plastic with at least one curved surface. Light moves faster in air than it does through glass or plastic. So, when a beam hits a lens, it slows down. When a beam enters glass at an angle, the part of the beam that hits the glass first slows down sooner, making the beam turn. Once the beam hits the air again, it speeds up and completes the trajectory. In this way, a lens can focus the light from an object into an image of the object on the other side. Convex lenses (sometimes called positive lenses) which are thick in the middle and curve out on one or both sides, take the light beams and redirect them towards the centre. Convex lenses are also called converging lenses.
Concave, or diverging lenses, are thick at the edges and curve inward on one or both sides. Concave lenses take light beams and redirect them away from the centre. Concave lenses are used in things like TV projectors to make light rays spread out into the distance.
Plus, take a look at: