Question

Describe why the sound of an oncoming siren changes as it passes you on the street. Please include terms such as blue and red shift. What is one way this is used in astronomy?

Answer 1

When siren passes you on street, you hear the pitch of the siren change: as it approaches, the siren’s pitch sounds higher than when it is moving away from you. This change is a common physical demonstration of the Doppler effect.The Doppler effect describes the change in the observed frequency of a wave when there is relative motion between the wave source and the observer.

A stationary source that’s emitting waves in all directions with a constant frequency. Any person standing still near the source will encounter each wavefront with the same frequency that it was emitted.But if the wave source moves, the pattern of wavefronts will look different. In the time between one wave peak being emitted and the next, the source will have moved so that the shells will no longer be concentric. The wavefronts will bunch up (get closer together) in front of the source as it travels and will be spaced out (further apart) behind it

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source:Gillian Isoardi

Sameway for light waves, the frequency determines the colour we see. The highest frequencies of light are at the blue end ofthe visible spectrum; the lowest frequencies appear at the red end of this spectrum.If stars and galaxies are travelling away from us, the apparent frequency of the light they emit decreases and their colour will move towards the red end of the spectrum. This is known as red-shifting.A star travelling towards us will appear blue-shifted(higher frequency).

Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth. For example the spectral lines emitted by hydrogen gas in distant galaxies is often observed to be considerably redshifted.