The Doppler Effect

The sound that changes as it passes

You have heard it a hundred times. An ambulance races toward you with its siren screaming high — neeee — then the instant it passes, the note drops to a lower naaaw and fades away. The driver never touched the siren. So why did the pitch change?

The answer is the Doppler effect: when a wave source moves relative to you, the frequency you receive changes even though the source is producing the same note all along. It is named after Christian Doppler, who described it in 1842.

Squeezing and stretching waves

A siren sends out sound waves in all directions, like rings spreading on a pond. If the source sits still, the rings are evenly spaced and everyone hears the true pitch.

Now let the source move. As it travels toward you, it chases after its own waves on your side, bunching them into a shorter wavelength. Shorter waves arriving means more per second, so a higher frequency — a higher pitch.

On the far side, the source is running away from its waves, stretching them into a longer wavelength. Fewer waves per second reach a listener behind it, giving a lower frequency — a lower pitch.

That is the whole effect. The waves still travel through the air at the normal speed of sound; the source has simply rearranged how the wavelengths are spaced. Because the wave equation says v = f λ, and v stays fixed in air, a shorter λ automatically means a higher f.

Toward = higher, away = lower

A useful rule:

• Source approaching → waves bunched → shorter λ → higher frequency (higher pitch / bluer light)
• Source receding → waves stretched → longer λ → lower frequency (lower pitch / redder light)

The bigger the relative speed, the bigger the shift. A slow bicycle bell barely changes; a Formula 1 car howling past produces a dramatic drop.

Worked example: a passing horn

A train horn sounds a steady 500 Hz. As it speeds toward a waiting passenger, the waves are bunched so tightly that the received frequency rises to about 540 Hz. After the train passes, the stretched waves arrive at about 465 Hz.

Shift while approaching: 540 − 500 = +40 Hz higher Shift while receding: 500 − 465 = −35 Hz lower Total jump heard at the moment of passing: 540 − 465 = 75 Hz

That 75 Hz drop is the sudden fall in pitch your ear notices the instant the train flashes by. (The approaching shift is slightly bigger than the receding one — a real feature of the Doppler maths.)

Doppler in light: red shift and blue shift

Light is a wave too, so it shows the Doppler effect — but instead of pitch, the colour shifts.

• A light source moving away has its waves stretched toward longer, redder wavelengths: a red shift.
• A source moving toward you is squeezed toward shorter, bluer wavelengths: a blue shift.

In the 1920s, Edwin Hubble found that the light from almost every distant galaxy is red-shifted, and the farther the galaxy, the bigger the shift. The conclusion was extraordinary: the galaxies are nearly all rushing away from us, which means the whole universe is expanding. The Doppler effect turned a roadside curiosity into one of the foundations of modern cosmology.

Try it yourself! 🧪 (safe version)

You do not need a speeding car to hear the Doppler effect.

1. Find an open outdoor space and a friend.
2. Have your friend put a loud, steady-toned buzzer or a phone playing a constant note inside a soft sock (so it cannot fly out and break).
3. Ask them to swing it in a wide, gentle circle on a length of string, well away from people and windows.
4. Stand a safe distance back and listen. As the buzzer swings toward you the pitch rises; as it swings away it falls — a steady wow-wow-wow.

You are hearing the wavelength being squeezed and stretched in real time. The same waves, the same speed of sound, just rearranged by motion. If you want to explore how those sound waves reach your ear in the first place, see the ear and how we hear.