The distance between two adjacent points on a wave that are in phase, e.g. crest to crest.

State the wave equation.

v = fλ, where v is the wave speed, f the frequency and λ the wavelength.

Which quantity stays the same when a wave passes from one medium to another?

The frequency, which is set by the source. The speed and wavelength change.

Describe a transverse wave.

The oscillations are perpendicular to the direction of energy transfer, e.g. water waves, light.

Describe a longitudinal wave.

The oscillations are parallel to the direction of energy transfer, forming compressions and rarefactions, e.g. sound.

What is the displacement of the molecules at the centre of a compression?

Zero. The pressure is a maximum there.

What does a displacement–distance graph show?

A snapshot of the wave at one instant, from which wavelength and amplitude are read.

What does a displacement–time graph show?

The motion of one point, from which period and amplitude are read.

How do points in adjacent loops of a stationary wave compare in phase?

They are in antiphase. Points within one loop are in phase.

Core practical: why measure across many wavelengths when finding the speed of sound?

The uncertainty in locating each in-phase position is fixed, so a larger distance gives a smaller percentage uncertainty in λ.

How is the speed of sound found from a graph of λ against 1/f?

The graph is a straight line through the origin and its gradient is the speed v.

Wave properties and wave graphsEdexcel A-Level Physics: Flashcards

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Define amplitude of a wave.

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Define amplitude of a wave.: The maximum displacement of a point on the wave from its equilibrium position.
Define wavelength.: The distance between two adjacent points on a wave that are in phase, e.g. crest to crest.
State the relationship between frequency and period.: f = 1/T.
State the wave equation.: v = fλ, where v is the wave speed, f the frequency and λ the wavelength.
Which quantity stays the same when a wave passes from one medium to another?: The frequency, which is set by the source. The speed and wavelength change.
Describe a transverse wave.: The oscillations are perpendicular to the direction of energy transfer, e.g. water waves, light.
Describe a longitudinal wave.: The oscillations are parallel to the direction of energy transfer, forming compressions and rarefactions, e.g. sound.
What is the displacement of the molecules at the centre of a compression?: Zero. The pressure is a maximum there.
What does a displacement–distance graph show?: A snapshot of the wave at one instant, from which wavelength and amplitude are read.
What does a displacement–time graph show?: The motion of one point, from which period and amplitude are read.
How do points in adjacent loops of a stationary wave compare in phase?: They are in antiphase. Points within one loop are in phase.
Core practical: why measure across many wavelengths when finding the speed of sound?: The uncertainty in locating each in-phase position is fixed, so a larger distance gives a smaller percentage uncertainty in λ.
How is the speed of sound found from a graph of λ against 1/f?: The graph is a straight line through the origin and its gradient is the speed v.

Exam questions on Wave properties and wave graphs

A student displays the signal from a microphone on an oscilloscope. The time base is set to 0.50 ms per division and the Y-gain to 2.0 V per division. One complete cycle of the trace spans 4.0 divisions horizontally, and the distance from a trough to the next crest spans 5.0 divisions vertically. The speed of sound in air is 340 m s⁻¹.
Calculate the wavelength of the sound wave.2 marks
A technician sends a sound wave along a long air-filled tube using a loudspeaker. She is interested in how the pressure of the air and the displacement of the air molecules from their equilibrium positions vary as a progressive wave passes along the tube.
Explain how the wave transfers energy along the tube although there is no net movement of air along it.2 marks
A student determines the speed of sound in air. A loudspeaker driven by a signal generator at 2000 Hz is placed at one end of a metre rule. Two microphones are connected to the two channels of an oscilloscope. Microphone 1 stays next to the loudspeaker and microphone 2 is moved slowly along the rule. The student records each position of microphone 2 at which the two traces are exactly in phase. The distance between the first and the sixth in-phase positions is 0.850 m.
Calculate the speed of sound in air from the student's results.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).