Happy Birthday, Christian Doppler
On this date in 1803, Christian Doppler was born in Salzburg, Austria. The son of a stone mason, Doppler became a mathematician and physicist. He is known for his explanation of the phenomenon called the Doppler effect.
When a tr
ain is approaching you and blowing its whistle, the whistle sounds higher-pitched than it does as it passes you and starts moving away. That’s because of the Doppler effect.
Most of us have the opportunity to hear the Doppler effect in action when an emergency vehicle approaches and passes us—the siren seems higher as it nears us, then slides to a lower pitch as it passes and recedes.
Listen to the Doppler effect on a car horn here.
Why does the Doppler effect occur?
As an ambulance approaches us, emitting the siren at the same steady pitch (or frequency), the the ambulance’s motion puts it closer to the sound wave that is traveling away from us. In other words, the sound waves bunch up in the direction that the ambulance is moving, and stretch out behind the ambulance. The bunched-up waves hit our ears more often, so the sound is higher. The stretched-out waves hit our ears at a lower frequency and sound lower.
Some animations on this site shows how it works.
And this animation actually shows an ambulance and should be easier for young children to understand.
Note that the sound being emitted doesn’t change in pitch—it just SEEMS to change to you, the listener, standing still by the road. If you were IN the ambulance, the sound would be steady and unchanging. If both you and the ambulance were still, the siren would again sound like a steady pitch.
The Kettering University website also has (lower down on the webpage) an animation to explain a sonic boom. A sonic boom is made when a plane travels faster than sound—the plane passes us before the sound reaches us, and all the bunched-up sound waves sound like a single thump. (Well, okay, a double thump, one for the nose and one for the tail of the plane. Usually the two thumps are so close together, most people hear them as one.)
Here is another Doppler-effect animation—and this time, YOU control the direction and speed of the movement. Try moving it slowly at first to see the waves bunch up and stretch out. Then try moving it quickly to see what happens when a plane moves faster than sound.
And here is a YouTube video about sonic booms. Did you know that you can SEE as well as HEAR a plane break the sound barrier?
The Doppler Effect in Astronomy
Christian Doppler first explained the phenomenon of the Doppler effect, not for sound, but for light waves. He had been studying binary stars; as they circle each other, each star seems to approach us slightly and then recede again, rhythmically. We see the approaching and receding motions as shifts in the star’s spectrum; as a star comes closer, its light is blue-shifted, and as it recedes from us, it’s red-shifted. 
The Doppler effect is one tool we use to find out about the structure, history, and future of the universe—including the facts that there was a Big Bang and that, billions of years from now, the Andromeda galaxy will collide with our Milky Way galaxy!
