The Doppler Effect: Why Ambulance Sirens Change Pitch

The Doppler Effect is the change in pitch you hear when a sound source moves toward or away from you.
As an ambulance approaches, sound waves are compressed, making the siren sound higher.
As it moves away, the waves are stretched, making the pitch drop lower.
The effect happens because sound waves pile up in front of a moving source and spread out behind it.
It’s not the siren itself that changes, but your perception of the frequency.
The faster the source moves, the stronger the Doppler shift you hear.
Astronomers use the same principle with light waves to measure if stars and galaxies are moving toward or away from Earth.
A redshift means light waves are stretched (moving away), while a blueshift means they’re compressed (moving closer).
The Doppler Effect is also used in radar guns to measure car speeds.
From speeding ambulances to distant galaxies, the Doppler Effect reveals the motion hidden in waves.