Define focus and principal focus.

Define the terms: principal focus

Define the principal focus of concave mirror.

Prove that for spherical mirrors ,the product of the distances of the object and the image from the principal focus is always equal to the square of the principal focal length.

Define principal focus of a mirror.

What do you mean by a lens? What are concave and convex lenses? What do you mean by Prnicipal axis, Optical centre and Principal focus and focal length of a lens?

Define the terms: 1 Optical centre 2.Principal axis 3. principal focus of lens.

An object is held at the principal focus of a concave lens of focal length F. where is the image formed?