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 converging (convex) or diverging (concave).
A converging lens is thicker in the middle than it is at the edges.
Parallel light rays that enter the lens converge.
They come together at a point on the principal axis called the principal focus F.
The centre of the lens is called the optical centre C.
A ray of light incident at the optical centre passes straight through without being bent.
In a ray diagram, a converging lens is drawn as a vertical line with outward facing arrows to indicate the shape of the lens.