Convex mirror is used as rear view mirror as it gives a clear diminished and an erect image of the traffic that is behind you. Also, has a large field of view i.e. it covers a wide range of the traffic behind you and gives a very clear image.Just imagine the fact that you are using a concave mirror. Everything behind you will be enlarged and tilted from the original position and you can’t see the actual traffic behind you. Even a plain mirror is not used as it does not cover a wide surface for the traffic.Hence, a convex mirror is used.