Polarisation Cambridge International AS & A Level Physics revision
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In plain words
Light is a transverse wave: its oscillations are at right angles to the way it is travelling, and they can point in any direction round the beam. A polarising filter lets through only one of those directions, like a slot that only lets through cards held flat. Sound can't be polarised, because its oscillations are along the beam, not across it.
Three things to know
- In an unpolarised transverse wave the oscillations are in many planes at right angles to the direction of travel. In a plane-polarised wave they are in one plane only.
- Only transverse waves can be polarised. Polarisation is the evidence that light is a transverse wave.
- A polarising filter transmits only the oscillations in one direction. A second filter turned at 90° to the first ("crossed") lets nothing through.
Tips and tricks
- If a question asks for evidence that a wave is transverse, the answer is that it can be polarised.
- Turning one filter through a full circle in front of another gives two bright positions and two dark ones.
It lands in your notebook with its questions as flashcards.
Polarisation: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Which waves can be polarised?
There must be oscillations across the direction of travel.
Two polarising filters are crossed at 90°. How much light gets through both?
The second filter blocks everything the first lets through.
What does polarisation show about light?
Longitudinal waves cannot be polarised.
Which of these cannot be polarised?
Sound is longitudinal.
How are the oscillations arranged in plane-polarised light?
The filter has removed every other direction.
Quiz
5 questions
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Worksheet
3 questions, 5 marks. Write your answers on paper, then check them.
Polarisation
Cambridge International AS & A Level Physics 9702 · 5 marks · papermunch.org
Name ______________________________ Date ______________
Explain why sound waves cannot be polarised.[2]
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Sound is a longitudinal wave: its oscillations are along the direction of travel, so there is no plane of oscillation to select.
Light passes through two polarising filters. Describe what is seen as the second filter is slowly turned through 180°.[2]
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The brightness changes. It is greatest when the two filters are lined up, falls to zero when they are at 90° to each other, and rises back to its greatest by 180°.
State what is meant by plane-polarised light.[1]
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Light whose oscillations are in one plane only.
Answers: Polarisation
- 1. Sound is a longitudinal wave: its oscillations are along the direction of travel, so there is no plane of oscillation to select.
- 2. The brightness changes. It is greatest when the two filters are lined up, falls to zero when they are at 90° to each other, and rises back to its greatest by 180°.
- 3. Light whose oscillations are in one plane only.