Question

A ray of light is incident on an equilateral glass prism placed on a horizontal table. For minimum deviation, which of the following is true?

(A) QR is horizontal
(B) PQ is horizontal
(C) Either PQ or QS is horizontal
(D) RS is horizontal

Answer 1

When the light travels from one medium to another (with different refractive indexes) it’s direction of propagation changes, in other words it is said to be deviated from its original path. The $\delta$gives deviation of light, and is defined as the angle between the incident and the emergent ray.

Complete step by step solution:
When the light travels through a prism, it is deviated two times during its course. First deviation occurs when it is incident on the prism, at point Q. The angle made by incident ray to the normal from the surface is represented by i, this ray is refracted inside by the glass prism.

The refracted ray comes out through the point R, and the angle of emergence represented by e. If the incident and the emergence rays are extended , they intersect at a point. The one of the angles produced by the intersection gives the angle of deviation.
At the condition of minimum deviation,
$\angle i = \angle e$
This can only be reached when QR is parallel to AB. This will make the emergence and incident angles equally inclined from the prism, and the light will undergo minimum deviation. The other conditions for minimum deviation can also be derived using the geometry of the prism or the prism formula.

Thus, option (A) is correct.

Note:
The angle of incidence can have any value ,but for minimum deviation both of these quantities must be equal, and as another result for this, the line QR should be horizontal. Also, it is to be noted that light gets deviated two times when it travels inside a prism.