# A. A ray of light

Given: -

Minimum deviation, = 30o

Prism is equilateral

Formula: -

The Refractive index of the prism in terms of minimum deviation is found as,

Where, is called the minimum deviation, and A is called the angle of the prism.

Calculation: -

Hence,

Also, we know that,

So,

Conclusion: -

the velocity of light in the given prism is, 2.12 × 108 m/s.

B) For emerging ray to graze along face AC,

TIR must occur on the face AC, so,

Hence, rAC = 45o

As, Q = R = 90o,

A + N = 180o

As, A = 60o , N = 180o – 60o = 1200

Also,

Applying Snell’s law on face AB,

Hence for grazing incidence on face AC,

SiniAB=√2× sin(15o) =1.414× 0.2588=0.366

SiniAB=0.366

iAB=21o (approx.)

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