Lenses are the most used things in optical devices like microscopes and telescopes. Bi-convex and bi-concave lenses are the most popular ones in use among school labs. Lenses use the phenomenon of refraction of light to form images.

The geometric centre of a lens is called its optic centre. The line passing through the optic centre and perpendicular to the plane of the lens is the principal axis. A light ray incident on a lens, after refraction appears to emanate from the principal focus in the case of a concave lens and passes through the focus in the case of a convex lens. The diameter of the lens gives the measure of its aperture. Read more at https://www.learnnext.com/lesson/CBSE-X-Science-Refraction-by-Spherical-Lenses.htm