Question

If mirror is so rotated through 20° , by what angle reflected ray on mirror rotates???​

Answer 1

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