The unmistakable sound of a fast car moving by, mimicked by kids the world over. Two similar but differently pitched noises are made by the vehicle, depending on whether it’s coming towards you or moving away from you. If the car was stationary, you would just hear the same noise all the time, so how does this happen?
The phenomenon of this change in pitch was first noticed by a German, Dr Doppler, and so in true scientist style, it’s known around the world as the Doppler Effect. Ah scientists, always happy to slap their name on a physical effect.
But back to the noises. Imagine the car moving along the road as a little creator of sound waves. The car goes along in this bubble and the noise always sound the same when you're inside.
But from the outside, depending on where you’re standing, sometimes the waves are being “pushed” towards you by the motion of the car, and sometimes they are being “pulled” away.
This means that the waves are either “squashed” or “stretched” when they reach your ear. Because the information in a sound wave is all held in the shape of the wave, changing the wavelength (the measure of how far apart each wave is) changes this information. So anything travelling quickly towards you will have a higher pitch (smaller wavelength) than it naturally would, and anything travelling quickly away from you will have a lower pitch (bigger wavelength).
Incidentally, this is also how they can tell if things in the universe are moving towards us or away from us because the light waves behave in exactly the same way!