Question

Which one of the following arrangements of optical components can be used to distinguish between unpolarised light and circularly polarised light? 

• A. (diagram with $\lambda/2$ plate and analyser)
• B. (diagram with polariser, $\lambda/2$ plate, analyser)
• C. (diagram with $\lambda/4$ plate and analyser)
• D. (diagram with polariser, $\lambda/4$ plate, analyser)

Hint:

A $\lambda/4$ plate converts circular polarisation into linear polarisation, enabling detection with an analyser.

Answer 1

The Correct Option is C

Step 1: Behaviour of circularly polarised light.
Circularly polarised light becomes linearly polarised when passed through a $\lambda/4$ plate.

Step 2: Behaviour of unpolarised light.
Unpolarised light becomes linearly polarised only after a polariser. The $\lambda/4$ plate has no effect on it before the polariser.

Step 3: Use of analyser.
After the light becomes linearly polarised, the analyser helps check intensity variation by rotation. For circularly polarised light, the analyser shows a constant intensity after the $\lambda/4$ plate. For unpolarised light (after polariser), intensity varies as $\cos^2\theta$.

Step 4: Conclusion.
Only arrangement (D): polariser → $\lambda/4$ plate → analyser can distinguish between unpolarised and circularly polarised light.