Two mirrors are joined together along a common side,the planes of the mirrors separated by the angle è.If the joined mirrors can be used as a retroreflector when a beam of light strikes both surfaces (one after the other),what is the value of è?
A)30 $\degree$
B)45 $\degree$
C)90 $\degree$
D)135 $\degree$

Question

Quizplus · Accepted Answer

A retroreflector reflects light back in the direction it came from. This can only happen if the angle of incidence (angle between the incident beam and the normal to the mirror) equals the angle of reflection (angle between the reflected beam and the normal to the mirror). When the two mirrors are joined along a common side, the normal to one mirror is perpendicular to the normal of the other mirror. Therefore, the incident angle on the first mirror is è/2 and the angle of reflection off the second mirror is also è/2. Thus, the total angle of reflection is è/2 + è/2 = è. This means that the angle between the two mirrors must be 90 degrees, making the correct answer choice C.

Two Mirrors Are Joined Together Along a Common Side,the Planes °\degree°

Two Mirrors Are Joined Together Along a Common Side,the Planes $\degree$