In the case of a plane mirror, by whatsoever angle , and in whatsoever direction, if the incident ray is rotated by an angle , the reflected ray will be rotated by an angle ( - ).

 

Minus  means that the magnitude of the angle is , but ABOUT THE POINT OF INCIDENCE, the relflected ray is rotated in the sense opposite to the sense of rotation of incident ray. (clockwise and anticlockwise are two senses of rotation)