The Doppler Effect: GCSE Physics

What causes the Doppler effect?

Imagine a source that emits waves at a steady frequency. If the source is stationary, the waves spread out equally in all directions — equally spaced in front and behind.

Now suppose the source moves toward an observer. By the time each successive wavefront is emitted, the source has moved slightly closer to the observer. The wavefronts bunch up in the direction of motion, reducing the wavelength and increasing the frequency the observer detects.

Conversely, if the source moves away from the observer, successive wavefronts are emitted from positions progressively further away. The wavefronts spread out behind the source, increasing the wavelength and reducing the frequency.

The key relationship: frequency and wavelength are inversely related (for a constant wave speed): as wavelength decreases, frequency increases, and vice versa.

$$v = f \lambda$$

The wave speed (v) in the medium does not change — only the apparent frequency (f) and wavelength (λ) as perceived by the observer change.

How does the Doppler effect explain the sound of a passing ambulance?

This is the classic illustration at GCSE. As an ambulance approaches:

• The siren emits waves at a fixed frequency (e.g. 700 Hz).
• Because the ambulance is moving toward you, each successive wavefront reaches you slightly sooner than it would from a stationary source.
• The wavefronts are compressed — you hear a higher pitch (higher frequency) than the actual siren.

As the ambulance passes and moves away:

• Each successive wavefront is emitted from a position slightly further away.
• The wavefronts are stretched — you hear a lower pitch (lower frequency) than the actual siren.

The drop in pitch as the ambulance passes is the Doppler effect in everyday life. The driver of the ambulance hears the siren at its true frequency throughout, because they move with the source.

How does the Doppler effect apply to light?

Light is an electromagnetic wave, so the Doppler effect applies to it as well — though the change in frequency manifests as a change in colour (or more precisely, wavelength in the electromagnetic spectrum).

Stars and galaxies emit light at characteristic frequencies determined by their atomic composition. If the galaxy is receding (moving away from Earth), its light is redshifted — the spectral lines are shifted toward the red end of the spectrum compared to where they would appear from a stationary source.

What does redshift tell us about the universe?

In the early 20th century, Edwin Hubble observed that almost all distant galaxies show redshift — their light is shifted toward longer (redder) wavelengths. The further away a galaxy is, the greater its redshift, meaning the faster it is receding from Earth.

This observation (Hubble's Law) has two major implications:

1. The universe is expanding — galaxies are not simply flying through space; space itself is stretching, carrying galaxies with it.
2. The Big Bang — if the universe is currently expanding, running the clock backwards implies that all matter was once concentrated in an extremely small, hot, dense state. The Big Bang is estimated to have occurred approximately 13.8 billion years ago.

The Doppler effect applied to light from galaxies is therefore one of the key pieces of evidence for the Big Bang model of cosmology.

Worked example: interpreting a redshift observation

A galaxy emits light with a characteristic wavelength of 500 nm (green light) from a particular element. An astronomer observes the same spectral line at 550 nm.

Step 1: The observed wavelength (550 nm) is longer than the emitted wavelength (500 nm) — it has been shifted toward the red end of the spectrum: redshift.

Step 2: This redshift means the galaxy is moving away from Earth (the source is receding).

Step 3: The fractional shift = (550 − 500)/500 = 0.10 = 10%. Using Hubble's Law, this shift corresponds to a recession velocity and can give an estimate of the galaxy's distance. (The full calculation uses v = H₀d, which is beyond standard GCSE but the qualitative reasoning is GCSE-level.)

Conclusion: The galaxy is receding from Earth, consistent with the expanding universe.

Frequently asked questions

Does the Doppler effect only occur when the source is moving?

No — the Doppler effect occurs whenever there is relative motion between the source and the observer. If the source is stationary and the observer moves toward the source, the observer encounters wavefronts more frequently, also raising the detected frequency. In practice, GCSE questions focus on the moving-source case (e.g. vehicles, stars), but the physics is symmetric: it is the relative motion that matters, not which object is "really" moving.

Why do we hear a sudden drop in pitch rather than a gradual change?

The pitch does change gradually as the ambulance approaches (it is at its highest just as the ambulance is closest) and again as it recedes. However, because a vehicle typically passes an observer very quickly on a road, the transition from approach to recession happens over a short time and sounds like a sudden drop. If you were moving at the same speed as the ambulance (e.g. on a motorway), you would hear the siren at its true frequency the entire time — no Doppler shift relative to you.

What is the difference between Doppler redshift and cosmological redshift?

Doppler redshift is caused by a source moving through space away from an observer — the waves are physically stretched by the motion. Cosmological redshift (the type relevant to distant galaxies) is caused by the expansion of space itself between the source and the observer — the wavelength is stretched as the space it travels through expands. At low recession velocities the two produce similar results and can both be described with the Doppler framework, but for very distant galaxies cosmological redshift requires general relativity to interpret accurately. At GCSE, the distinction is not required — you should know that galactic redshift implies recession and an expanding universe.

How is the Doppler effect used in medicine and speed cameras?

Ultrasound Doppler scans in medicine: a handheld probe emits ultrasound waves that reflect off moving red blood cells. The frequency shift of the reflected waves tells the machine the velocity of blood flow. This is used to detect blockages, measure foetal heart rate, and assess arterial health. Speed cameras (radar guns): a device emits microwaves or radio waves at a known frequency. The waves reflect off a moving vehicle and return at a shifted frequency. The shift is proportional to the vehicle's speed relative to the camera.

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