5 May, 13:59

# 3. A ray of light incident on one face of an equilateral glass prism is refracted in such a way that it emerges from the opposite surface at an angle of 900 to the normal. Calculate the i. angle of incidence. ii. minimum deviation of the ray of light passing through the prism [n_glass=1.52]

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1. 5 May, 14:10
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i) angle of incidence; i = 29.43°

ii) δm = 38.92°

Explanation:

Prism is equilateral so angle of prism (A) = 60°

Refractive index of glass; n_glass = 1.52

A) Let's assume the incident angle = i and Critical angle = θc

We know that, sin θc = 1/n

Thus;

sin θc = 1/n_glass

θc = sin^ (-1) (1/n_glass)

θc = sin^ (-1) (1/1.52)

θc = 41.14°

Now, the angle of prism will be the sum of external angle that is critical angle and reflected angle.

Thus;

A = r + θc

r = A - θc

So;

r = 60° - 41. 14°

r = 18.86°

From, Snell's law. If we apply it to this question, we will have;

(sin i) / (sin r) = n_glass

Where;

i is angle of incidence and r is angle of reflection.

Let's make i the subject;

i = sin^ (-1) (n_glass * sin r)

i = sin^ (-1) (1.52 * sin 18.86)

i = sin^ (-1) 0.4914

i = 29.43°

B) The formula to calculate minimum deviation would be from;

μ = [sin ((A + δm) / 2) ] / (sin A/2)

Where;

μ is Refractive index

δm is minimum angle of deviation

A is angle of prism

Now Refractive index is given by a formula; μ = (sin i) / (sin r)

So; μ = (sin 29.43) / (sin 18.86)

μ = 1.52

Thus;

1.52 = [sin ((60 + δm) / 2) ] / (sin 60/2)

1.52 * sin 30 = sin ((60 + δm) / 2)

0.76 = sin ((60 + δm) / 2)

sin^ (-1) 0.76 = ((60 + δm) / 2)

49.46 * 2 = (60 + δm)

98.92 - 60 = δm

δm = 38.92°