The maximum displacement of a point on the wave from its equilibrium position.

The shortest distance along a wave between two points that are oscillating in phase.

Define the period of a wave.

The time taken for one complete oscillation of a point on the wave.

State the wave equation.

c = fλ (wave speed = frequency × wavelength).

A wave has frequency 50 Hz and wavelength 0.20 m. What is its speed?

c = fλ = 50 × 0.20 = 10 m s⁻¹.

What determines the speed of a wave: the source or the medium?

The medium (for a string, its tension and mass per unit length). Changing the source frequency changes the wavelength, not the speed.

What does it mean for two points on a wave to be in phase?

They are at the same stage of the oscillation cycle: same displacement and moving in the same direction at the same time.

What is the phase difference between points in antiphase?

π rad (180°), or an odd multiple of π; they always move in opposite directions.

Two points are 0.25 m apart on a wave of wavelength 1.0 m. What is the phase difference?

0.25 of a cycle, which is π/2 rad (90°). The further point lags.

How is the phase difference between two points related to their separation?

Phase difference = (separation ÷ wavelength) × 2π rad (or × 360°).

Progressive wavesAQA A-Level Physics: Flashcards

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What does a progressive wave transfer?

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What does a progressive wave transfer?: Energy, through oscillation of particles or fields, without the medium itself travelling along with the wave.
Define amplitude.: The maximum displacement of a point on the wave from its equilibrium position.
Define wavelength.: The shortest distance along a wave between two points that are oscillating in phase.
Define the period of a wave.: The time taken for one complete oscillation of a point on the wave.
State the relationship between frequency and period.: f = 1/T
State the wave equation.: c = fλ (wave speed = frequency × wavelength).
A wave has frequency 50 Hz and wavelength 0.20 m. What is its speed?: c = fλ = 50 × 0.20 = 10 m s⁻¹.
What determines the speed of a wave: the source or the medium?: The medium (for a string, its tension and mass per unit length). Changing the source frequency changes the wavelength, not the speed.
What does it mean for two points on a wave to be in phase?: They are at the same stage of the oscillation cycle: same displacement and moving in the same direction at the same time.
What is the phase difference between points in antiphase?: π rad (180°), or an odd multiple of π; they always move in opposite directions.
Convert a phase difference of one quarter of a cycle into radians and degrees.: π/2 rad and 90°.
Two points are 0.25 m apart on a wave of wavelength 1.0 m. What is the phase difference?: 0.25 of a cycle, which is π/2 rad (90°). The further point lags.
How is the phase difference between two points related to their separation?: Phase difference = (separation ÷ wavelength) × 2π rad (or × 360°).

Exam questions on Progressive waves

A student uses a vibrating dipper to generate straight water waves in a ripple tank. The dipper vibrates at 12 Hz and the student measures the distance between adjacent wave crests on the water surface as 2.5 cm.
The student doubles the frequency of the dipper. The speed of the waves is unchanged because the depth of water is unchanged. Calculate the new wavelength.2 marks
Two buoys, A and B, float on a lake in line with the direction in which a sinusoidal water wave is travelling. The wave has frequency 0.50 Hz and wavelength 8.0 m. Buoy B is 3.0 m further along the direction of travel than buoy A.
Calculate the time by which the motion of buoy B lags behind that of buoy A.2 marks
A loudspeaker produces a continuous pure note of frequency 850 Hz in air, where the speed of sound is 340 m s⁻¹. Two small microphones are placed in a straight line with the loudspeaker, at distances of 1.20 m and 1.50 m from it.
Calculate the wavelength of the sound and the phase difference, in radians, between the signals at the two microphones.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).