Any experimental evidence of Doppler shift in light?

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Discussion Overview

The discussion centers around the experimental evidence for the Doppler shift of light, specifically regarding how light frequency changes when an object moves towards or away from an observer. Participants explore both theoretical and practical aspects of this phenomenon, including its implications in various contexts such as astronomy and radar technology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that there are many experiments demonstrating the Doppler shift of light, including personal experiences from college.
  • One participant mentions the use of police radar traps as an example of Doppler shift, clarifying that radar uses radio waves, which are a form of light.
  • Another participant questions the measurability of frequency differences in light at relatively low speeds, suggesting that high-speed objects like stars should not be visible if their light is significantly redshifted.
  • Participants discuss the mechanics of measuring slight frequency differences, using analogies involving sine waves to illustrate how small shifts can be detected.
  • One participant describes an experimental method involving the measurement of the solar spectrum from different limbs of the sun to observe the Doppler effect due to the sun's rotation.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and agreement regarding the practical implications and measurements of the Doppler shift in light. While some agree on the existence of experimental evidence, others raise questions about the visibility and detection of light from fast-moving objects, indicating that multiple views remain in the discussion.

Contextual Notes

Participants highlight the challenges in measuring small frequency shifts in light compared to sound, and the dependence on specific experimental setups. The discussion also reflects on the limitations of detecting shifts in light from astronomical objects due to their high velocities.

Crazy Tosser
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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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Many, many experiments. I even did this in college. It's not hard.
 
Ever been caught by a police radar trap?
 
Excuse me, I always thought police trap was sound waves, not light waves. And how was the experiment performed, Vanadium?
 
Crazy Tosser said:
Excuse me, I always thought police trap was sound waves, not light waves.
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.
 
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?
 
Crazy Tosser said:
Ok, so a 30 mph difference in speed produces a measurable difference in frequency of light?
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.

You would think that stars that travel at thousands km/sec away from us would not even be seen?
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.
 
Crazy Tosser said:
And how was the experiment performed, Vanadium?

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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