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Doppler Effect

A2 Phys Ph4

Carl Bater

on 23 October 2015

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Transcript of Doppler Effect

4) The distance between the star and the planet, if the star has a mass of 2 x 10 kg.
How to Measure the Speed of an Orbiting Body.
As an object moves towards you, any waves it produces are squashed together, causing an increase in frequency for the oncoming waves. As it moves away from you, the waves are stretched apart, causing a decrease in frequency.
Doppler Effect in Light…
Edwin Hubble (the man the telescope is named after) noticed that if you look at other distant galaxies the absorption lines in their spectra appear to have been shifted towards the red end of the spectrum. He explained this using the Doppler effect – the light from those galaxies is being stretched out. This means that they MUST be moving away from us –and also must be moving at a high speed.
The Maths Bit
The distortion in wavelength produced by a moving source of waves is directly proportional to its speed.
This holds true for velocities up to about 10% of c
Velocity at right angles to the observer produces little or no Doppler shift.

By convention, the wavelength shift will be positive if v is positive, so we measure v away from the observer.
Applying this to Orbits
As stars orbit around the common centre of mass they share with thier planets the light they emit is shifted a small amount.
By measuring this shift and comparing it to a known stationary wavelength, we can determine enough information to begin applying Keplers Laws in order to evaluate it fully.
Light from a distant star in a system shown in the diagram below reaches Earth.
An astronomer records how an absorption line at 650 nm varies over a long time period.
Using this data, find:
1) The time period of the star around the centre of mass
2) The velocity of the star around the centre of mass.
3) The radius at which the star orbits the centre of mass.
5) The mass of the planet.
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