3.1.4Waves

The Doppler Effect — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 14

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision

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Part 1 of 7What the Doppler effect is

When a source of waves moves relative to an observer, the frequency that the observer detects is different from the frequency that the source emits.

Doppler effect

This is the Doppler effect.

The common example is a police siren: its pitch is higher as it approaches you, and lower as it moves away,

while the driver hears the steady pitch because there is no relative motion between the siren and driver.

Part 2 of 7The explanation using wavefronts

A stationary source sends out wavefronts that spread as circles centred on the source, spaced one wavelength apart in every direction.

The explanation using wavefronts

If the source moves, each wavefront is emitted from a point further along its path.

In front of the source the wavefronts are bunched together,

so the observed wavelength is shorter and, since the wave speed in the medium is fixed, the observed frequency is higher.

Behind the source the wavefronts are stretched apart, so the wavelength is longer and the frequency lower.

The explanation using wavefronts

Source approaching: observed frequency higher, wavelength shorter.

Source receding: observed frequency lower, wavelength longer.

Source at rest: observed frequency equals emitted frequency.

The speed of the wave does not change, because it depends only on the medium. It is the spacing of the wavefronts that changes.

Part 3 of 7Moving observers

The effect also occurs when the observer moves and the source is still. If you move towards a source, you meet more wavefronts each second, so you detect a higher frequency. If you move away, you meet fewer, so the frequency is lower. In both cases what matters is the relative motion of source and observer.

Part 4 of 7Doppler effect for light and applications

Light shows the same effect. Light from a source moving away is shifted towards longer wavelengths, which is called red-shift. Astronomers measure the red-shift of light from distant galaxies and find that nearly all are shifted to the red, showing that the galaxies are moving away from us and that the Universe is expanding. Other uses include:

Doppler effect for light and applications

Radar speed guns, which send out radio waves and detect the shift in frequency of the reflected waves from a moving car.

Medical ultrasound that measures blood flow speed by detecting the shift in frequency of reflected waves from moving blood.

Weather radar that detects the motion of rain clouds.

The larger the speed of the source, the larger the shift in frequency.

Part 5 of 7Limits and links

For everyday speeds the change in frequency is small compared with the original frequency for light, but for sound it is noticeable because the speed of sound is only about 340 m s⁻¹.

Limits and links

The effect links with the idea of the wave equation (The Wave Equation),

because wavelength and frequency vary inversely when the wave speed is unchanged.

It also links to the electromagnetic spectrum (Electromagnetic Waves), since red-shift is a shift towards the long wavelength end.

Part 6 of 7Estimating the size of the shift

For sound from a source moving towards a stationary observer at speed v_s, the observed frequency is f' = f v / (v − v_s), where v is the speed of sound.

Estimating the size of the shift

You do not need to use this at IGCSE, but it shows how the effect depends on speed.

For a 700 Hz siren on a vehicle moving at 20 m s⁻¹ in air where v = 340 m s⁻¹, the approaching frequency is about 700 × 340 / 320 = 744 Hz,

and the receding frequency is about 700 × 340 / 360 = 661 Hz.

The difference of over 80 Hz is clearly audible.

For light, the speed is so large that even at high speeds the shift in wavelength is small, but for galaxies moving at a significant fraction of the speed of light it becomes measurable.

Estimating the size of the shift

Spectral lines from hydrogen appear at longer wavelengths than the same lines measured in a laboratory.

The further away a galaxy is, the greater its red-shift, which is evidence that the Universe is expanding, and supports the Big Bang model.

Part 7 of 7Worked examples

An ambulance siren emits 700 Hz. Describe what a stationary observer hears as the ambulance approaches and then passes.
Light from a distant galaxy is observed to have a longer wavelength than the same light measured in a laboratory. What does this suggest?

Key terms

Term	Meaning
Doppler effect	Change in observed frequency due to relative motion of source and observer
Red-shift	Shift of light to longer wavelengths from a receding source
Wavefront	Line joining points in phase
Observed frequency	Frequency detected by the observer
Relative motion	Motion of the source compared with the observer

Avoid these mistakes

Saying the siren itself changes pitch for the driver – no relative motion, no change.

Believing the speed of the sound changes – it is the wavelength and frequency.

Saying that an approaching source has a longer wavelength.

Linking red-shift to the colour of the star rather than its motion.

Thinking the Doppler effect only happens for sound.

Last updated: 20 October 2026