0Principal focus:- The principal focus of a concave mirror is defined as the point on its principle axis to which all the incident rays parallel and close to the axis converge after reflection through the mirror. It is represented by letter ‘F’ and lies always in front of a concave mirror, thus a real focus.
Focal length:- The focal length of a concave mirror is defined as the distance between the pole and principle focus of a concave mirror. It is represented by letter ‘f’ and is always taken as negative.
