Discussion
The plane of polarisation of a plane polarized light rotates by 60° after passing through a wave plate. The pass-axis of the wave plate is at an angle \(\alpha\) with respect to the plane of polarisation of the incident light. The wave plate and \(\alpha\) are
Answer: B
The plate is \(\lambda/2\), with \(\alpha = 30^\circ\).
A half-wave plate reflects the plane of polarisation about its axis: light polarised at angle \(\alpha\) to the axis comes out at \(-\alpha\), a rotation of \(2\alpha\). A rotation of \(60^\circ\) therefore needs \(\alpha = 30^\circ\).
A quarter-wave plate does not rotate a plane of polarisation; in general it produces elliptically polarised light, so both \(\lambda/4\) options fail. A pass-axis at \(120^\circ\) is the same line as one at \(-60^\circ\); it turns the plane by \(-120^\circ\), which lands on the same line only because a plane of polarisation is defined to within \(180^\circ\). The key takes the direct reading, the smallest axis angle giving a \(60^\circ\) rotation, \(30^\circ\).
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