Discussion
Which one of the following arrangements of optical components can be used to distinguish between an unpolarised light and a circularly polarised light?
[In each option the light passes through the listed components in order, from left to right.]
Answer: C
The arrangement is light, \(\lambda/4\) plate, analyser.
A quarter-wave plate turns circularly polarised light into plane polarised light, because it adds a further \(\pi/2\) between the two components that were already \(\pi/2\) apart. Rotating the analyser then gives complete extinction at one position. Unpolarised light remains unpolarised after the plate, so its intensity through the analyser stays constant. The two can therefore be told apart.
A half-wave plate turns circular light into circular light of the opposite hand, so the analyser shows no variation for either beam. Putting a polariser first destroys the difference, since both beams become plane polarised before reaching the plate.
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