Question

Define focus?

Answer 1

Principal focus:

1. The incident rays coming parallel to the principal axis after reflection appear to converge to a common point on the principal axis, this point is called the principal focus of a concave mirror.
2. The incident rays coming parallel to the principal axis after appearing to converge to a common point on the principal axis, this point is called the principal focus of a convex mirror.
3. It is usually denoted by F.
4. Radius of curvature is observed to be equal to twice the focal length for spherical mirrors with small apertures. Hence $R=2f$