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Question
The Figure below shows a ray of monochromatic light LM incident on the first surface AB of a regular (equilateral) glass prism ABC. The emergent ray grazes the adjacent surface AC. Calculate the angle of incidence. (Refractive Index of glass = 1.5)
Solution
Given A = 60°, n = 1.5, At the surface AC, r2 is the critical angle
∴
`sin r_2 = 1/n`
= `1/1.5`
= `2/3`
= 0.6667
∴ r2 = sin-1 (0.6667)
= 41.8°
Also, r1 + r2 = A
r1 = A - r2
= 60° - 41.8°
= 18.2°
At the surface AB,
`sini/(sinr_1) = n`
sin i = 1.5 × sin 18.2°
= 1.5 × 0.31
= 0.465
∴ i = sin-1 (0.465)
= 27.7°
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