Question

How could you distinguish experimentally, by only using a linear polariser and a detector, beams of plane polarised light, circularly polarised light and elliptically polarised light ?

Answer 1

plane polarised light light vector vibrate along a fixed straight line in a plane.

circularly polarised light light vector rotate with constant magnitude in a plane perpendicular to the direction of propagation

elliptically polarised light magnitude of light vector varies periodically during its rotation in a plane perpendicular to the direction of propagation.

Experimental set-up

Place your polarised light on left end an linear polariser (called analyser) after it and detector at right most on the same line. (See fig)

Detection of

Plane polarised light If you rotate the analyser then if the intensity of emerging light detected at detector varies with zero minimum then the light is plane polarised light. The variation can be determined by Malus law.

Circularly polarised light

Now replace the plane polarised light with circularly polarised light. Now if you rotate the analyser then intensity of the detected light shows no variation.

Then the light would be either unpolarized light or circularly polarised.

To confirm it is passed through a quarter wave plate which produce a phase difference of π/2 between ordinary and extraordinary wave.the circularly polarised light after passing through quarter plate becomes plane polarised.

Now if the analyser rotated then light shows variation in intensity with zero minimum then the light must be circularly polarised.

Elliptically polarised light now if you detect thetelliptically polarised light through rotating analyser you will observe the variation in intensity but minimum intensity will not be zero.

To confirm adjust the analyser for maximum intensity. Introduce another quarter plate between polarised light and analyser such that optic axis of plate is parallel to the principal section of the analyser. The optic axis of the plate is now become parallel to major axis of elliptical vibration. The light after passing through the quarter plate become plane polarised.

If analyser is now rotated the detected intensity will vary with zero minimum.