Question

why concave mirror is used as a shaving mirror

Answer 1

A concave mirror forms virtual, erect and magnified image of the object PROVIDED THAT THE OBJECT IS BETWEEN FOCUS AND POLE OF THE MIRROR !! A person who is shaving always keeps his face close to the mirror so that his face comes between pole and focus. Thus he is able to view an enlarged and erect image of his face.

If his face is beyond focus, it will form real and inverted image.

Answer 2

because in concave mirror we can see an enlarged image of the face. They reflect light so that the image formed is of exactly the same size as the object.