Types of lens

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).

Converging (or convex) lenses

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.

A convex lens with four light rays going through. On the other side of the lens, the rays turn inwards and all meet at the centre which is marked with a small circle.

In a ray diagram, a converging lens is drawn as a vertical line with outward facing arrows to indicate the shape of the lens.

Key point

  1. The focal length f of a converging lens is the distance between the optical centre, C, of the lens and the principal focus, F.
  2. Focal length is measured in m, cm or mm.
Four light rays pass through the lens. The lines turn in and meet on the other side of the lens.