Question

Define focus of lens.

(Don't copy from google)​

Answer 1

Answer:

The focus is the point, or plane, at which light rays from infinity converge after passing through a lens and traveling a distance of one focal length. In a refractor the first lens through which light from a celestial object passes is called the objective lens

Answer 2

Answer:

Principal focus of a lens is defined as the point through which a parallel of light after refraction passes through in case of convex lens and appears to pass thrugh in case of concave lens.