01) Principal focus : The principle focus of a convex mirror is the point on its principal axis to which a beam of light rays, initially parallel and close to the axis, appears to diverge after being reflected from the mirror. A convex mirror has a virtual focus and lies always behind the mirror. It is represented by letter F.
2) Focal length : The focal length of a convex mirror is the distance between its pole and principle focus, it is represented by letter f, and is always taken as positive.
