Define a longitudinal wave.

A wave in which the oscillations are parallel to the direction of energy transfer.

Give two examples of transverse waves.

Electromagnetic waves (including light) and waves on a string.

What are the regions of a longitudinal wave called?

Compressions, where particles are closer together, and rarefactions, where they are further apart.

What is the speed of all electromagnetic waves in a vacuum?

3.00 × 10⁸ m s⁻¹, whatever the frequency or wavelength.

What oscillates in an electromagnetic wave?

Electric and magnetic fields, at right angles to each other and to the direction of travel.

What does plane polarised mean?

The oscillations are restricted to a single plane perpendicular to the direction of travel.

Why can sound waves not be polarised?

They are longitudinal, so the oscillations are parallel to the direction of travel and there are no perpendicular oscillations to restrict to one plane.

Two polarising filters are crossed. What is transmitted?

No light: the second filter blocks the plane polarised light from the first because its axis is at 90°.

How many times is the transmitted intensity zero in one full rotation of the second of two filters?

Twice (at 90° and 270°), with maxima twice (at 0° and 180°).

What does polarisation of light provide evidence for?

That light is a transverse wave.

How do polarising sunglasses reduce glare?

Reflected light is polarised mainly horizontally; the lenses have a vertical transmission axis so they absorb most of it.

Why must a receiving rod aerial be aligned with the transmitting aerial?

The signal is strongest when the rod is parallel to the electric field of the polarised wave, and almost zero when perpendicular to it.

Longitudinal and transverse waves and polarisationAQA A-Level Physics: Flashcards

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Define a transverse wave.

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Define a transverse wave.: A wave in which the oscillations are perpendicular to the direction of energy transfer.
Define a longitudinal wave.: A wave in which the oscillations are parallel to the direction of energy transfer.
Give two examples of transverse waves.: Electromagnetic waves (including light) and waves on a string.
Give an example of a longitudinal wave.: Sound waves in air.
What are the regions of a longitudinal wave called?: Compressions, where particles are closer together, and rarefactions, where they are further apart.
What is the speed of all electromagnetic waves in a vacuum?: 3.00 × 10⁸ m s⁻¹, whatever the frequency or wavelength.
What oscillates in an electromagnetic wave?: Electric and magnetic fields, at right angles to each other and to the direction of travel.
What does plane polarised mean?: The oscillations are restricted to a single plane perpendicular to the direction of travel.
Why can sound waves not be polarised?: They are longitudinal, so the oscillations are parallel to the direction of travel and there are no perpendicular oscillations to restrict to one plane.
Two polarising filters are crossed. What is transmitted?: No light: the second filter blocks the plane polarised light from the first because its axis is at 90°.
How many times is the transmitted intensity zero in one full rotation of the second of two filters?: Twice (at 90° and 270°), with maxima twice (at 0° and 180°).
What does polarisation of light provide evidence for?: That light is a transverse wave.
How do polarising sunglasses reduce glare?: Reflected light is polarised mainly horizontally; the lenses have a vertical transmission axis so they absorb most of it.
Why must a receiving rod aerial be aligned with the transmitting aerial?: The signal is strongest when the rod is parallel to the electric field of the polarised wave, and almost zero when perpendicular to it.

Exam questions on Longitudinal and transverse waves and polarisation

A student lays a long, loosely coiled spring along a bench. She moves one end of the spring rapidly back and forth along the length of the spring, producing regions where the coils are pushed closer together and regions where they are pulled further apart. These regions travel steadily along the spring away from her hand.
Explain why the wave on the spring cannot be plane polarised.2 marks
A communications satellite in geostationary orbit transmits television signals as microwaves to a receiving dish on the ground directly below it. The satellite is at a height of 3.6 × 10⁷ m above the dish. The speed of electromagnetic waves in a vacuum is 3.00 × 10⁸ m s⁻¹, and you may assume the whole path is a vacuum.
The satellite also carries an infrared sensor that sends an infrared pulse towards the dish at the same instant that a microwave pulse is sent. Deduce which pulse arrives first at the dish, giving a reason.2 marks
A student investigates polarising filters (Polaroid sheets). She places a filter P1 in the path of unpolarised light from a lamp, and a second filter P2 behind it that can be rotated about the axis of the light beam. A light sensor behind P2 measures the intensity of the light transmitted.
Describe and explain how the sensor reading changes as P2 is rotated slowly through 360°.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).