Question

Three glass prisms $A,B$ and $C$ of the same refractive index are placed in contact with each other as shown in figure, with no air gap between the prism. A monochromatic ray of light $OP$ passes through the prism assembly and emerges as $QR$. The condition of minimum deviation is satisfied in the prism

• A. $A$ and $C$
• B. $B$ and $C$
• C. $A$ and $B$
• D. $A,B$ and $C$

Answer 1

The correct option is C $A$ and $B$

For minimum deviation produced by the prisms, the refracted ray becomes parallel to the base of the prism after getting refracted through $1$st refracting face.
From the diagram, this condition is achieved for prism $A$ and $B$.

Alternate solution:

Since for minimum deviation in a prism following two conditions need to be satisfied,

$1.$ $i=e$ i.e the angle of incidence should be equal to the angle of emergence.

$2.$ $r_1=r_2$ i.e. the angle of refraction of the two refracting surfaces of the prism have to be equal.

So, these conditions are satisfied by the prisms $A$ and $B$.

Why this question? It tests your understanding of basic definition of minimum deviation.