Answer
a). (4) $(alpha-beta)$
b). $20^{\circ}$
Work Step by Step
a). Angle BAO = $90^{\circ}-beta$
Angle ABO = $180^{\circ}-[(90^{\circ}-beta)+alpha]=90^{\circ}+beta-alpha$
Thus, angle of incidence of beam from the second mirror = $90^{\circ}-(90^{\circ}+beta-alpha)=(alpha-beta)$
Also, angle of reflection = angle of incidence.
Option (4) is the answer.
b). Angle of reflection of the beam from the second mirror = $(alpha-beta)$
$60^{\circ}-40^{\circ}=20^{\circ}$
