Compound lens' focal length

Description

A lens is a transmissive optical device which affects the focusing of a light beam through refraction. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually along a common axis.
A compound lens is a collection of simple lenses of different shapes and made of materials of different refractive indices, arranged one after the other with a common axis. If “thin” lenses are placed in contact, then the combined focal length of the lenses is related to its one of the focal lengths.