A ray of monochromatic light is passing through an equilateral...

For an equilateral prism, the angle of the prism is $A = 60^\circ$.

It is given that the refracted ray inside the prism is parallel to its base. This implies that the prism is in the position of minimum deviation.

In the position of minimum deviation, the angle of emergence $e$ is equal to the angle of incidence $i$.

Given $i = 50^\circ$, we have $e = 50^\circ$.

The angle of deviation $\delta$ is given by the relation:

$\delta = i + e - A$

Substituting the values, we get:

$\delta = 50^\circ + 50^\circ - 60^\circ$

$\delta = 100^\circ - 60^\circ = 40^\circ$

Answer: $40^\circ$