Question

A given ray of light suffers minimum deviation in an equilateral prism P. Additional prisms Q and R of identical shape and of same material as that of P are now combined as shown in the figure. The ray will now suffer

• A. greater deviation
• B. no deviation
• C. same deviation as before
• D. total internal reflection

Hint:

When you see an even number of identical prisms alternating (up-down-up-down), the net deviation is zero. When you have an odd number (like 3 here), the net deviation is usually that of a single prism.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept
Prism combinations can either add to the total deviation or cancel it out. When two identical prisms are placed in an opposite orientation (one upright, one inverted), they act like a parallel-sided glass slab.

Step 2: Detailed Explanation
1. Prisms Q and R together form an inverted prism structure that is identical in shape to prism P. 2. When prism Q and R are combined with P in the configuration shown, the base of the first prism is opposite to the base of the middle section. 3. Effectively, the first two prisms (P and Q) act as a parallel glass slab which causes zero net deviation (only a lateral shift). 4. The final ray then passes through the third prism (R). Since the previous combination resulted in no angular deviation, the ray enters the last prism with the same relative orientation as it entered the first. 5. Consequently, the net deviation of the entire system remains equal to the deviation produced by a single prism.

Step 3: Final Answer
The ray will suffer the same deviation as before.