The Doppler Effect: what does motion do to waves?

The Doppler Effect is when motion changes how waves look or sound to you.

Imagine you're on a playground, and your friend is riding a bike toward you while ringing a bell. As they come closer, the ringing sounds louder and the pitch goes up, it's like the bell is saying "Hi!" in a higher voice. When they pass by and start going away, the sound gets softer and the pitch goes down, like the bell is now saying "Bye!" in a lower voice.

This happens because of how waves work. If your friend is moving toward you, each wave from the bell reaches you a little faster, making the sound seem higher. When they move away, the waves stretch out more, so the sound feels lower.

Think about a fire truck with its siren on. As it zooms past you, the sound changes from high to low, just like your friend on the bike! The same idea happens with light too: stars moving toward us look slightly bluer, and those moving away look redder.

So whether it’s a bell, a fire truck, or even a star, motion plays tricks on waves, and that's the Doppler Effect in action.