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Any experimental evidence of Doppler shift in light?

  1. Dec 11, 2008 #1
    Were there any experiments that proved doppler shift of light on a fast moving object - that it has higher frequency if the object is moving towards you and lower if the object is moving away?
     
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  3. Dec 11, 2008 #2

    Vanadium 50

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    Many, many experiments. I even did this in college. It's not hard.
     
  4. Dec 11, 2008 #3

    mgb_phys

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    Ever been caught by a police radar trap?
     
  5. Dec 11, 2008 #4
    Excuse me, I always thought police trap was sound waves, not light waves. And how was the experiment performed, Vanadium?
     
  6. Dec 11, 2008 #5

    mgb_phys

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    Police radar guns use radar (ie radio waves = light) they measure the difference in frequency between the outgoing signal and the doppler shifted return. It's very easy in electronics to accurately measure small frequency differences between two signals - much easier than measuring pulse time of flight for instance.

    You can do an easy experiment on acoustical doppler shift by just spinning a buzzer around your head on a wire. Optical / radio needs a bit more equipment but it's still an ugrad lab practical.
     
  7. Dec 11, 2008 #6
    Ok, so a 30 mph difference in speed produces a measurable difference in frequency of light? >.< You would think that stars that travel at thousands km/sec away from us would not even be seen?
     
  8. Dec 11, 2008 #7

    mgb_phys

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    There is a trick to measure slight differences in two frequencies.
    Imagine you have a very long sin wave drawn on a chart - now add another very long wave with a tiny difference in frequency and put it next to the first. They will line-up almost perfectly at the start.
    But if they differ by 1 part in a million then after a 1/2 million waves the top of one will line up with the bottom of another - this is very easy to detect.
    Since the rader gun is sending billions of wavelengths /second it only has to sample a fraction of a second to line up a million waves.

    Speed of light is 300,000km/s so 3000km/s is only a 1% shift. Stars are a continuum source so as 1% of the visible light is moved into the IR another 1% of the UV is moved into the visible. You do see the effect in very distant galaxies - they are red shifted out of the visible into the infrared.
     
  9. Dec 11, 2008 #8

    Vanadium 50

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    Measuring the spectrum of the limb of the sun that is moving away from us, and comparing it to the limb of the sun that is moving towards us. The sun rotates at a few km per second, so it's about a 10 ppm effect. It's quite straightforward, actually.
     
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