How to answer questions on polarisation in IB Physics

This mini tutorial focusses only on questions relating to polarisation in IB Physics (Topic 4:3 – Wave Characteristics).

You may have seen it? I sent out a survey about which topics most students struggled with in IB Physics and the overwhelming majority of responses listed Topic 4: Waves as one of their most troubling topics.

One particular equation from Laura asks:

“Hi Sally! Could you go through some polarization problems next time? I seem to understand the theory very well but always get the questions wrong... ”

— Laura

Polarisation is a concept that can be easily understood, but questions are tricky to answer. So I’ll show 6 exam-style questions related to polarisation below and give you model answers for each one.


Exam-Style Question 1: What is meant by polarisation of light waves?

Answer: Polarised light is when the oscillations / vibrations (associated with the wave) are in one plane only. OR polarised light is when the electric field oscillations / vibrations are in one direction only.


Exam-Style Question 2: Explain why sound waves cannot be polarised

Answer: Sound is a longitudinal wave (i.e not transverse wave) and oscillations are in one direction already.


Exam-Style Question 3: Some sunglasses are described as ‘polaroid’. What does this mean and what advantages do they offer over other types of sunglasses?

Answer: Unpolarised light from the sun can be partially polarised by non-metal reflective surfaces. The reflected light is partially polarised in the plane of the surface. The sunglasses have polaroid filters at 90 degrees to some of the reflected light and therefore cut out some of the reflected light. This has the advantage of increasing the contrast and allowing the wearer to see more clearly, without glare.


Exam-Style Question 4: Three polarisers are arranged below. The transmission axis of each filter is shown as a double-headed arrow. Shapes A and B show the areas where the three polarising filters overlap. Shade areas A and B with an appropriate ‘darkness’ to show how much light is blocked in this arrangement.

Answer:


Exam-Style Question 5: The diagram shows two identical polarising filters and an unpolarised light source. The arrows indicate the plane in which the electric field of the wave oscillates. If polarised light is reaching the observer, draw the direction of the transmission axis on the second filter on the diagram below.

Answer: The transmission axis must also be vertical, because the light does not decrease intensity after the second polarising filter.


Exam-Style Question 6: An aerial transmits vertically polarised radio waves in the direction shown by the double-sided arrow. The waves are detected by a receiving aerial, which sits horizontally opposite the transmitting aerial.

The receiving aerial is rotated through 180° about the axis joining the centre of the two aerials. Plot a graph of the intensity of radio waves received by the aerial against the angle through which the aerial rotates, from 0° through to 180°.

Answer:


If you’d like to delve deeper into these problems – please watch the video at the top of this post.

Hope it helps!

Photo by:
Adam Adams

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