Answer:
Look at the mirror from the positive axis and suppose the incident ray is at angle at 40 from the y-axis
Incident ray at 50 deg
reflected ray at 130 deg
total angle of deflection = 130 - 50 = 80 deg
Now rotate the mirror by 20 deg
incident ray at 30 deg
reflected ray at 150 deg (angle of incidence must equal angle of reflection)
total angle of deflection = 150 - 30 = 120 deg
the difference between 2 reflected rays = 120 - 80 = 40 deg
So if you increase (decrease) the incident angle the reflected ray will be increased by twice the angle of rotation of the mirror - or the perpendicular to the mirror increases by X degrees the angles of incidence and reflection will each increase by X deg
