The Doppler Effect

Doppler
Christian Doppler 1803 - 1853
Christian Doppler 1803 – 1853

It’s time for me to return to my first love in science, some hard core physics! Maybe those three words should never be placed together, but I do so because I am going to explain an effect that no doubt everyone will have heard on many occasions in their lifetime, but one which many would be unable to explain themselves – the Doppler Effect.

The most common instance of the Doppler Effect is with emergency vehicle sirens. We’ve all been walking in the street and heard a distant siren, let’s say from an ambulance, and as it gets closer it gradually gets louder and louder and the pitch builds higher and higher, till the ambulance is whizzing directly past you. Then the moment the siren speeds past you the pitch immediately drops away and lowers till the ambulance is out of sight.

Now, the reason for this gradual build then sudden drop is all to do with how sound waves behave when being produced from a source – like an ambulance siren – that is moving relative to an observer – like someone walking in the street. Sound waves like to travel in a sine wave with peaks and troughs, much like the waves you see at the beach. Waves from a siren would be produced at regular intervals, with the distance between too peaks known as its wavelength. But if the ambulance is moving towards you as it is producing these sound waves, then the waves start to pile up and the distance between each wave becomes squashed. The result is that what you hear is the pitch of the siren gradually getting higher and higher as the ambulance approaches.

Once the siren has passed the observer then the opposite happens. As it speeds away the sound waves consistently take a little longer to reach you, and become more and more stretched as the ambulance moves away from you.

The wavelenght getting smaller as the abulance moves toward an observer, but stretches as the ambulance moves away from the observer
The wavelength gets smaller as the ambulance moves toward an observer, but stretches as the ambulance moves away from the observer

The Doppler Effect can be used in many applications, from measuring blood flow in medicine to roadside radars that catch speeding drivers, but one of its best uses has been in astronomy. It can be used to discover planets that are orbiting distant stars, as light from these planets can show Doppler shifts. As the planet moves towards us the frequency rises and the light waves become squashed together, and as it spins away the light frequency drops and the light waves stretch apart. Light from an approaching planet is said to be ‘blue shifted’, and as it moves away it is said to be ‘red shifted’. This is because blue light has a smaller wave length compared to red light.  This little shift in frequency can be spotted as a planet passes in front of a star, and has been used to discover hundreds of planets since the 1990’s.

How the blue shift and red shift of stars can help discover distant planets
How the blue shift and red shift of stars can help discover distant planets

Red shifts can also arise from the expansion of the universe itself, at which point it gets called a cosmological red shift. If the space between us and a distant galaxy swells steadily as the universe expands, it is similar to the galaxy moving away from us with some speed.  As a result the light from the galaxy is shifted to lower frequencies because the waves must travel further and further to reach us. Therefore very distant galaxies look redder in appearance than galaxies much closer to us.

So there you have it, the Doppler Effect explained as best I can. And if you are still struggling then I refer you to Sheldon Cooper of the Big Bang Theory…..

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