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# A ray of light passes through an equilateral prism such that the angle of incidence is equal to the angle of emergence and each one is equal to $\large\frac{3}{4}$ th the angle of prism. Then angle of deviation is :

$\begin {array} {1 1} (1)\;45^{\circ} & \quad (2)\;39^{\circ} \\ (3)\;20^{\circ} & \quad (4)\;30^{\circ} \end {array}$

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A)
$(4)\;30^{\circ}$

Comment
A)
i + e = A+ d

i --->angle of incidence

e--->emergence

A--->angle of prism

d---->angle of deviation

i=e= (3/4)A

(3/4)A+(3/4)A=A+d

1.5 A-A=d

d=A/2

A=60 degree (equilateral prism)

d=30 ..