Thin Lenses and Image Formation

Convex and concave lenses at work.

A lens refracts light at both of its surfaces. A convex lens brings parallel rays together at its principal focus; a concave lens spreads them out as if they came from a focus on the incoming side.

With a convex lens, an object beyond twice the focal length gives a real, inverted, smaller image. Move the object inside the focal length and the image becomes virtual, upright and magnified, which is how a simple magnifying glass works.

• Convex lens: converging; concave lens: diverging.
• Real images can be shown on a screen; virtual ones cannot.
• Rays through the optical centre go straight on.

Browse all 50 physics and electronics pages  |  Back to the course index