The Doppler Effect

Have you noticed that an ambulance siren sounds higher as it comes toward you and lower as it drives away? This change in pitch is called the Doppler Effect. It happens because the movement of the siren squishes sound waves together in front and stretches them out behind.

How It Works with Sound

When a siren is coming toward you, each sound wave is sent from a closer position. The waves get squeezed together, making the pitch higher. When the siren moves away, each wave is sent from farther away. The waves get stretched out, making the pitch lower. The siren itself does not change its sound.

The Doppler Effect with Light

The Doppler Effect also works with light. When a star moves toward Earth, its light waves get squeezed and look bluer. When a star moves away, its light waves stretch and look redder. Astronomers use this to figure out if galaxies are moving toward us or away from us.

Fun Facts

• The Doppler Effect was named after Austrian scientist Christian Doppler, who described it in 1842.
• Police radar guns use the Doppler Effect to measure how fast cars are going.
• Astronomers discovered that the universe is expanding by observing the Doppler Effect in starlight.

Did You Know?

Bats use the Doppler Effect to hunt. They send out sound waves and listen for the echoes. If an insect is flying toward the bat, the echo pitch changes, helping the bat track its prey!