Convex and concave lenses

Jonny Nelson introduces an animated explanation of lenses

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.

Convex lenses

A convex 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 called the principal focus.

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 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.

Four light rays pass through the lens. The lines turn in and meet on the other side of the lens.

Concave lenses

A concave lens with four rays of light passing through. On the other side of the lens, the rays all turn away from the centre.

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.

Four rays of light, go through the lens. On the other side of the lens the rays of light turn away from the centre.