1996 General Studies, all questions
UPSC Prelims 1996 · General Studies
Question 116: Science and Technology
When a mirror is rotated by an angle of θ, the reflected ray will rotate by
a0°
bθ / 2
cθ
d2θ
AnswerThe answer is (d).
When a mirror turns by an angle, the normal turns by the same angle, so for a fixed incident ray the angle of incidence changes by that angle and the reflected ray turns by twice as much: 2 theta. Remember: rotating a mirror by theta turns the reflected ray by 2 theta, the principle used in the mirror galvanometer.
Why each option is right or wrong
- (a) The reflected ray does turn when the mirror turns.
- (b) Not the relation: reflection doubles the turn, not halves it.
- (c) Would hold only if reflection did not double the turn.
- (d) Right: the reflected ray rotates by 2 theta.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 1996, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.