A ray of light is incident on a plane mirror at an angle of 40° with respect to surface normal. When it gets reflected from the mirror, it undergoes a deviation of
- A.40°
- B.100°
- C.90°
- D.80°
Correct answer
Explanation
The correct answer is B, 100°. For reflection at a plane mirror, the deviation is 180° − 2i, and with i = 40° this gives 180° − 80° = 100°.
By the law of reflection, the angle of reflection equals the angle of incidence, both measured from the normal, so the incident and reflected rays are 2i = 80° apart. If the ray went straight on it would not deviate at all; the reflected ray turns back, so its change of direction is the supplement of 80°, that is 100°. At normal incidence (i = 0°) the ray retraces its path and the deviation is the maximum, 180°.
A is wrong because 40° is just the angle of incidence, not the change in direction.
C is wrong because 90° deviation would need an angle of incidence of 45°.
D is wrong because 80° is the angle between the incident and reflected rays, not the deviation.
Exam tip: plane mirror deviation δ = 180° − 2i; angle between incident and reflected rays = 2i.
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