State the unit of intensity in terms of watts, and in base units.

W m⁻², which is kg s⁻³ in base units.

What happens to the intensity when the distance from a point source is doubled?

It falls to one quarter (inverse square law).

What are the assumptions of the inverse square law for a source?

The source radiates uniformly in all directions and no energy is absorbed by the medium.

What is meant by a plane polarised wave?

A transverse wave whose oscillations are restricted to one plane, perpendicular to the direction of travel.

Why can sound not be polarised?

It is longitudinal: its oscillations are along the direction of travel, so there is no perpendicular plane to restrict.

What does a polarising filter do?

It transmits oscillations parallel to its transmission axis and blocks those perpendicular to it.

How do polarising sunglasses reduce glare?

Reflected glare is mostly horizontally polarised and the lenses have a vertical transmission axis, so most of it is blocked.

What is seen if a receiving rod aerial is rotated through 360° in front of a polarised radio transmitter?

Two maxima and two minima of signal, showing that radio waves are plane polarised and transverse.

Intensity and polarisationEdexcel A-Level Physics: Flashcards

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Define the intensity of a wave.

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Define the intensity of a wave.: The power incident per unit area, perpendicular to the direction of travel of the wave: I = P/A.
State the unit of intensity in terms of watts, and in base units.: W m⁻², which is kg s⁻³ in base units.
What is the area of a sphere of radius r?: 4πr².
State the formula for the intensity at distance r from a point source of power P.: I = P/(4πr²).
What happens to the intensity when the distance from a point source is doubled?: It falls to one quarter (inverse square law).
What happens to the intensity when the distance is tripled?: It falls to one ninth.
What are the assumptions of the inverse square law for a source?: The source radiates uniformly in all directions and no energy is absorbed by the medium.
What is meant by a plane polarised wave?: A transverse wave whose oscillations are restricted to one plane, perpendicular to the direction of travel.
Which types of wave can be polarised?: Only transverse waves.
Why can sound not be polarised?: It is longitudinal: its oscillations are along the direction of travel, so there is no perpendicular plane to restrict.
What does a polarising filter do?: It transmits oscillations parallel to its transmission axis and blocks those perpendicular to it.
How do polarising sunglasses reduce glare?: Reflected glare is mostly horizontally polarised and the lenses have a vertical transmission axis, so most of it is blocked.
What is seen if a receiving rod aerial is rotated through 360° in front of a polarised radio transmitter?: Two maxima and two minima of signal, showing that radio waves are plane polarised and transverse.

Exam questions on Intensity and polarisation

A small lamp in a large dark hall behaves as a point source. It radiates 12 W of light uniformly in all directions, and none of the light is absorbed by the air.
State what is meant by the intensity of a wave and explain why the intensity of the light falls as the distance from the lamp increases.2 marks
An engineer tests solar panels outdoors. At midday the intensity of sunlight, measured perpendicular to the surface of the panels, is 1000 W m⁻². The first panel has an area of 1.6 m² and an efficiency of 18%, meaning that 18% of the incident power is converted to electrical power.
A second panel has an efficiency of 15% and must also deliver 288 W in the same sunlight. Calculate the area it needs.2 marks
A fisherman wears polarising sunglasses. Light reflected from the surface of a calm lake is partially plane polarised with its oscillations mainly horizontal. The lenses of the sunglasses have a vertical transmission axis.
Explain what is meant by plane polarisation and why sound waves cannot be plane polarised.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).