Waves - The Doppler Effect and Redshift

This GCSE Physics quiz about waves will challenge you on the Doppler effect and redshift. The Doppler effect is also known as the Doppler shift. It describes the change in the wavelength of a wave that is being emitted by a moving object. The idea was first suggested in 1842 by the Austrian scientist Christian Doppler. He knew that a ship meets waves at a faster rate (higher frequency) when it is sailing in the direction from which the waves are coming than if it was sailing in the same direction that the waves were travelling. He knew that both sound and light were waves so he reasoned that the same effect would be noticed. He also said that it didn't matter whether it was the source of the waves that was moving or the person observing them.

Evidence to support the Doppler effect came a few years later. A Dutch scientist (with the incredibly long name of Christophorus Henricus Diedericus Buys Ballot) used musicians and a train to test the idea.

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He used two groups of musicians, one group on a platform at a station and the other group on an open wagon on a train. Both groups played the same note. As the train steamed towards the station at full speed (about 40 miles per hour), the observers on the platform could hear that the note played by the musicians on the train was slightly higher pitched than the musicians on the platform and lower as it steamed away. The two notes matched each other as the train passed the observers.

It is easy to spot this effect for yourself these days - listen to the sound of a car going past, it's quite noticeable when you are at a motorway services. Before the British police and emergency services adopted American style sirens, emergency vehicles were fitted with two-tone sirens (and before that they had bells). It was always possible to hear from which direction they were approaching as the pitch of the sirens was higher or lower than normal. It is much more difficult to tell with modern sirens as they use a continually changing pitch so one second they sound like they are approaching and the next second it seems like they are moving away. The police use the Doppler effect to catch speeding motorists - their radar equipment measures the wavelength change of the returning reflections of the radar waves and calculates the speed of the targeted vehicle.

Doppler was more interested in light from the stars than in sound waves. He believed that his effect could be used to measure the movements of stars. He knew that the colour that you see depends on the wavelength of the light hitting your retina. If the star was approaching, he said the wavelength would be shorter and vice versa. Doppler made a few mistakes but that is no surprise, this was new science. He believed that all stars where the same colour (white or a pale yellow like the Sun) and if star was approaching it would appear redder. We now know that neither of these things are true but he was on the right track.

When you split starlight into a spectrum, it has a number of dark lines that correspond to the elements in the star or galaxy. We know from studying spectra in a laboratory where these should be. At the beginning of the twentieth century, astronomers studying light from distant stars and galaxies noticed that for more distant objects, these lines were displaced. This is called redshift if the lines are displaced towards the red end of the spectrum and blueshift if they are more towards the blue end. Redshift indicates the star or galaxy is receding (moving away from us) and blueshift indicates it is approaching. American astronomer Edwin Hubble is credited with the discovery that the more distant objects are, the faster they are receding from us and realised that we live in an expanding universe. This had in fact been worked out from Einstein's theory of relativity by a Belgian, Georges Lemaître 2 years before Hubble but for a variety of reasons, it was Hubble who took the glory!

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  1. Where does C.M.B.R come from?
    It has a temperature close to absolute zero
  2. What is the cosmic microwave background radiation (C.M.B.R.)?
    Scientists think that by studying the C.M.B.R, they can understand some of the conditions which existed in the early parts of our Universe's development
  3. It can be observed that there is an increase in the wavelength of light received from distant galaxies. The further away the galaxy is, the faster it is moving away from us and the larger the observed increase in wavelength. What is this effect commonly known as?
    It is known as redshift as the wavelengths of light received from objects travelling away from us are displaced towards the red end of the electromagnetic spectrum. Similarly, light-emitting objects travelling towards us have wavelengths which are shifted towards the blue end of the spectrum and thus this effect is known as blueshift
  4. What is the Doppler effect?
    Both the frequency and the wavelength change when a source is moving relative to an observer. You can hear this effect yourself by listening carefully to traffic going past on a main road. You can hear how the sound changes when the vehicles are travelling towards you and away from you
  5. How does the frequency and wavelength of a wave source change when it moves towards an observer?
    If the wavelength of a particular wave decreases, then the frequency will always increase as more waves will pass the observer each second
  6. How does the frequency and wavelength of a wave source change when it moves away from an observer?
    For light, the effect is only noticeable when the movement is at extremely high speeds
  7. If a police car is travelling towards an observer and its siren is on, will the frequency of the siren be higher, lower or the same as when the car is moving away from the observer?
    Successive waves will arrive sooner than if the car was stationary
  8. Why does redshift support the idea of the Big Bang theory?
    The further away the galaxies are, the faster they appear to be moving
  9. Which is the only current theory that explains and accounts for the existence of the C.M.B.R?
    This has become the accepted theory of the origin of our Universe but there are still many unanswered questions
  10. If a police car is travelling away from an observer and its siren is on, how will the wavelength be different to when the car is moving towards the observer?
    Successive waves will arrive later than they would if the car was stationary

Author: Martin Moore

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