# Any experimental evidence of Doppler shift in light?

1. Dec 11, 2008

### Crazy Tosser

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?

2. Dec 11, 2008

Staff Emeritus
Many, many experiments. I even did this in college. It's not hard.

3. Dec 11, 2008

### mgb_phys

Ever been caught by a police radar trap?

4. Dec 11, 2008

### Crazy Tosser

Excuse me, I always thought police trap was sound waves, not light waves. And how was the experiment performed, Vanadium?

5. Dec 11, 2008

### mgb_phys

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.

6. Dec 11, 2008

### Crazy Tosser

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?

7. Dec 11, 2008

### mgb_phys

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.

8. Dec 11, 2008