If your operating system is not capable of viewing this animation,
click (thin lens).

A thin lens is defined as a lens with a thickness that is approximately ignorable compared with the
radii of curvature of the lens. If a lens is thicker than that measure, the thin lens equation cannot be used.

Due to the geometrical symmetry, the thin lens equation can be used for a convex and concave mirrors.

Please note the following so you can understand how the thin len equation can be derived:

This derivation is not fundamental, but it is more intuitive.

If you identify two equal angles in two different triangles as shown,
the two triangles are similar. Namely, each correspondent side has the same ratio.

Waning! This animation uses sounds. Before you start, check the volume of your computer.

Here is an additional information. The object and image distances have to be taken from the
center of the lens as shown below. Then, you can prove the thin lens equation works.