Science Common Sense

225 - How does the Doppler effect work?

Imagine you're standing by the road, and a police car is coming towards you with its sirens on. As it gets closer, the sound of the siren gets higher and louder. When it passes you and moves away, the sound gets lower and softer.

That's because of the Doppler effect. When the police car is moving towards you, the sound waves get pushed together and become shorter. This makes the sound higher. When it's moving away, the sound waves spread out and become longer. This makes the sound lower.

It's like when you're on a train, and you blow a whistle. If the train is standing still, the sound goes out in all directions equally. But if the train is moving, the sound in front of you gets squished together, making it higher, and the sound behind you gets stretched out, making it lower.

The Doppler effect works with light too, not just sound. That's how scientists can tell if stars are moving towards or away from us by looking at the color of their light.