In a concave mirror, the reflecting surface is shaped such that when the rays of light strike the mirror they are reflected to converge at a point called the focal point. The point of focus of a concave mirror lies in front of the mirror and the focal length is considered positive.
In the case of a convex mirror the rays of light that strike the mirror are reflected in such a way that they diverge away from a point called the focal point. The focal point in this case lies behind the mirror. It is not a real point but an imaginary one. It is assumed to be the point from where the rays originate and is determined by extending the reflected rays of light in the reverse direction and estimating the point where they meet. The focal length of a convex mirror is considered to be negative.