How can a Doppler effect be attributed to light?

[falzer]

Laser gun speed detection (or lidar) measures the time-of-flight of an emitted and reflected pulse of light, not the frequency shift. It's not based on the doppler effect AFAIK.

Do this several times a second and you can quickly calculate an object's speed.

 

[epepke]

Laser gun speed detection (or lidar) measures the time-of-flight of an emitted and reflected pulse of light, not the frequency shift. It's not based on the doppler effect AFAIK.

That's quite correct.

However, radar guns do measure the frequency shift.

 

[I less than three logic]

Laser gun speed detection (or lidar) measures the time-of-flight of an emitted and reflected pulse of light, not the frequency shift. It's not based on the doppler effect AFAIK.

Do this several times a second and you can quickly calculate an object's speed.

Your right. Even says so in my own link.
Its just the radar gun that use the doppler effect, the lasers count the round-trip time in nanoseconds to calculate speed.

 

[Beausoleil]

If the speed of light is constant, regardless of whether it is being emitted from an object that is moving toward or away from an observer, I don‘t see how motion can have a Doppler effect on light as in the expanding universe theory. In other words, how can something that is not effected by motion, be effected by motion?

Suppose a light source is 1 light year away from you. Turn it on, one year later the wave front hits you. Between you and the light source there are N = (one year) * (frequency) oscillations of the wave, and the wavelength is (one light year)/N.

Same light source, but it starts moving away from you the instant it is turned on. Because c is constant, the wavefront still reaches you one year later. If the frequency were the same, there would still be (one year) * ( frequency) oscillations of the wave between you and the source, but the wavelength would be (one light year + distance moved in one year)/N. In other words, the wavelength would have changed.

So there must be a Doppler effect. But, since c is constant, wavelength can't change if frequency doesn't so it isn't quite as simple as that. Remember time dilation - clocks seem to run more slowly in a moving frame of reference. You measure the frequency of an object by timing a fixed number of oscillations. If you watched someone do this in a moving frame of reference, their clock would seem to be running more slowly. If you measured their oscillator against your clock, you would get a different answer. The frequency of light we see coming from an object in a moving rest frame is exactly analogous to this.

(But redshift in the expanding universe theory is different again. It's like my opening example, but rather than have the object move, you stretch the space between you and it.)

 

[Earthborn]

However, it's called the Doppler effect, for some strange reason that I can't make out.

It is called the Doppler effect because it is an effect proposed by Doppler, in his Ueber das farbige Licht der Doppelsterne und einiger andere Gestirne des Himmels (About the coloured light of double stars and other heavenly bodies). He assumed that all the colour differences of stars were caused by their movement towards or away from us.

It was the Dutch scientist Buys Ballot who realised that the effect would also apply to sound, and he set out an experiment where people with perfect pitch would try to determine the change in pitch of notes played by musicians on a train driving by. An experiment that proved very difficult as the notes played could be very difficult to hear because of the sound of the locomotive.

Buys Ballot managed to confirm that movement can indeed change the pitch of sound, and therefore also the colour of light. But he also concluded that movement could not be the main explanation for the colour of stars, because he figured that like the noise of the locomotive and the lack of discipline of the musicians, there would be too many confounding factors in the movement of stars to create a stable starcolour.

 

[Dr Adequate]

If the speed of light is constant, regardless of whether it is being emitted from an object that is moving toward or away from an observer, I don‘t see how motion can have a Doppler effect on light as in the expanding universe theory.

Please to remember that in any given medium the speed of sound is also a constant. Yet there is a Doppler effect for sound.

The Doppler effect is about wavelength, not velocity.

 

[Dr Adequate]

Now, there is something like a sonic boom that happens when there is a disturbance that goes faster than the nominal speed of light in a medium, but I fear that is beyond the fringe of audience comprehension.

Our minds are too puny to comprehend Cherenkov radiation?

 

[Soapy Sam]

Or to know the difference between "effects" and "affects"?

 

[Melendwyr]

Our minds are too puny to comprehend Cherenkov radiation?

In fairness, it's actually kind of a trippy concept.

But it's still pretty easy to understand: it's just the electromagnetic equivalent of a sonic boom.

 

[LordoftheLeftHand]

Or to know the difference between "effects" and "affects"?

To be honest, I gave up on that one a long time ago

LLH

 

[epepke]

It is called the Doppler effect because it is an effect proposed by Doppler, in his Ueber das farbige Licht der Doppelsterne und einiger andere Gestirne des Himmels (About the coloured light of double stars and other heavenly bodies). He assumed that all the colour differences of stars were caused by their movement towards or away from us.

It was the Dutch scientist Buys Ballot who realised that the effect would also apply to sound, and he set out an experiment where people with perfect pitch would try to determine the change in pitch of notes played by musicians on a train driving by. An experiment that proved very difficult as the notes played could be very difficult to hear because of the sound of the locomotive.

OK, thanks. I take it all back. The effect on light is the Doppler shift, and the goof is calling what happens to sound the Doppler shift, when it should be called the Ballot shift.

In any event, the math is different between a moving source of sound and a moving source of light, and so it's probably not a good idea to use the same word for both.

 

[epepke]

Our minds are too puny to comprehend Cherenkov radiation?

Not yours, but if one is still in the state of being confused by the Doppler shift of light, then it's probably a bit much to heap it on at this time.

 

[TobiasTheViking]

To be honest, I gave up on that one a long time ago

LLH

Can't be that hard.

Lets try.

And how did that effect you LordoftheLeftHand
And what was the affect of that LordoftheLeftHand

*me grins and runs away giggling*

Sincerely
Tubse
No, i really am not a 13 year old school girl... really

 

[LordoftheLeftHand]

Can't be that hard.

Lets try.

And how did that effect you LordoftheLeftHand
And what was the affect of that LordoftheLeftHand

*me grins and runs away giggling*

Sincerely
Tubse
No, i really am not a 13 year old school girl... really

Fingers in ears:

I'm not listening!

LLH