C.5.1—Doppler effect

Core idea

The Doppler effect is the observed change in frequency, and therefore usually wavelength, caused by relative motion between a wave source and an observer. When the source and observer approach, wavefronts arrive more frequently and the observed frequency is higher. When they separate, the observed frequency is lower.

Apply it to sound

For sound, the wave travels through a medium. A moving source changes the spacing of emitted wavefronts in the medium; a moving observer changes how quickly the observer meets the wavefronts. In either case, motion toward one another gives a higher observed frequency and motion apart gives a lower one. The source frequency itself has not changed merely because the observer hears a different frequency.

Apply it to light

The Doppler effect also occurs for electromagnetic waves. A source moving toward an observer produces a shorter observed wavelength and a higher frequency (blueshift); moving away produces a longer wavelength and lower frequency (redshift). Unlike sound, the measured speed of light remains $c$ ; the observed change is in frequency and wavelength.

Use the shift as a measurement

Medical Doppler ultrasound uses a frequency shift in reflected sound to infer blood-flow speed. Radar uses a shift in reflected microwaves to infer the radial speed of a vehicle, aircraft or storm. In both cases the detected shift is tied to motion toward or away from the receiver, not to a change in the emitted frequency at the source.

Check the boundary

Do not explain light Doppler shift by adding the source speed to $c$ . In this course the low-relative-speed approximation is used for the change in frequency or wavelength; the light speed remains $c$ .

How it is assessed

Questions explain a redshift/blueshift observation or compare the wavelength and speed received from a moving sound source.

Command terms

Explain / What is

What earns marks

State that relative motion changes the observed frequency/wavelength, identify approach as higher frequency or blueshift and recession as lower frequency or redshift, and keep sound speed and light speed conceptually distinct.

Watch for

Calling a light redshift a reduction in light speed, or reversing the approach/recession relationship between wavelength and observed frequency.

Representative question

[Maximum number: 1]

The diagram shows a train travelling in a straight line at constant speed v, as it approaches the platform of a station.

The whistle of the engine is emitting a sound of constant frequency. Which of the following is not true for the sound of the whistle heard by an observer on the platform?

A sudden change in frequency of the sound as the train passes the observer.

A sound of constant frequency as the train approaches the observer.

A sound of increasing frequency as the train approaches the observer and of decreasing frequency after the train has passed the observer.

A sound of constant frequency after the train has passed the observer.

C.5 Doppler effect is secure when you can connect relative motion to the observed wave.

Approach raises observed frequency; recession lowers it
A moving source compresses or spreads wavefront spacing
A moving observer changes encounter rate, not medium wavelength
For light at low relative speed, Δf/f=Δλ/λ≈v/c
Spectral-line shifts reveal motion of stars and galaxies

C.5.1—Doppler effect

Model the Doppler Effect

C.5.1 Exam Analysis

Assessment in practice

Retrieve the Core C.5 Doppler Effect Model

Question 1