Science On The Beach

If you ask a small child what noise an aeroplane makes they will probably go EEEEEEeeeeaaaaawwwwww, run around with their arms outstretched, and bump into something fragile and expensive. The interesting thing about this (aside from how small children always hit the most fragile and expensive thing around) is that even young kids know the imaginary aeroplane's noise changes as it flies towards, over, and then away from them. It must be a pretty basic part of life if even four year olds know about it, but most people don't even realise it has a name.

Well it does: It's called Doppler shift.

To understand Doppler shift (better than a four year old, and without running into things) we need to be able to think in terms of waves: Sound is a wave (a pressure wave if you want to impress someone) and how long or short those waves are determines what you hear: Shorter waves = higher pitch. Longer waves = lower pitch.

What does this have to do with aeroplanes going EEEEEEeeeeaaaaawwwwww? The best way to explain this is with a trip to the beach.

Don't have a beach handy? Then we'll have to imagine one: Golden sand, blue sky, sun, and calm blue sea. Out on the sea there is a ferry, making regular waves from its propeller - 1 a second or so. Imagine wading out into the water until the tops of those waves are just striking over your knees.

The rate which the wave tops hit you at is called their frequency, and the distance between wave tops is called their wavelength - so a longer wavelength means a lower frequency (because the gap between wave tops is longer), a shorter wavelength means a higher frequency (because the gap between wave tops is shorter).

What if the ferry comes towards you? The waves won't go any faster - a wave travels at just one, constant, speed. With the ferry coming up behind the waves, and the propeller still making a new wave every second, the waves between the ferry and you have to bunch up. So the distance between wave tops is shorter, and they hit your knees more often - their frequency is higher. If the ferry moves away from you then its propeller is still making a wave a second, but now the waves must stretch to fill the growing gap between ferry and shore. So the wavelength will be longer, and the frequency lower.

That's Doppler shift, and now we can apply it to sound waves coming from an aeroplane: If the gap between the approaching aeroplane and a listener is shrinking then the sound waves seem shorter, so the pitch is higher. As the aeroplane passes over head it is briefly not moving towards or away from the listener very much, and we hear the engine as it would be standing still. And as it moves away the gap between the plane and the listener is getting wider, so the sound waves seem to be stretched, and the pitch heard is lower. Put all that together and you get:

EEEEEEEEEEEEaaaaaaaawwwwwwwwwwwwwww!

The principle applies to any waves with a constant speed, including light waves - but with light waves it is the colour that changes with frequency.

Now, a quick challenge (this will only take a moment I promise): Do an internet search for 'Doppler shift uses'. How many uses for this effect do you find?