A horizontal ray of light passes through a prism of index 1.50 and apex angle 4º and then strikes a vertical mirror, as shown in the figure
A
. Through what angle must the mirror be rotated if after reflection the ray is to be horizontal? 
1 0
1 0
B
2 0
C
2.5 0
D
1.5 0
Text Solution
Verified by ExpertsThe correct answer is:
A
As the apex angle is very small ( α = 4 0 ), the angle of deviation & can be obtained approximately:
δ = (n – 1) α = (1.5 – 1) × 4 0 = 2 0 .
Fig.
From Fig. we see that if the reflected ray is to be horizontal, the mirror must be rotated clockwise through an angle γ given by
γ = 
= 1 0
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