How do we know electromagnetic waves are light?

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Per the maxwell equations, we know that em waves travel at the velocity of light, but that is not a sufficient condition to say that electromagnetic waves are light. How do we know that electromagnetic waves are light? They could just be something that has the same velocity as light.

Any insight is appreciated.
 

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  • #2
russ_watters
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Per the maxwell equations, we know that em waves travel at the velocity of light, but that is not a sufficient condition to say that electromagnetic waves are light. How do we know that electromagnetic waves are light? They could just be something that has the same velocity as light.

Any insight is appreciated.
I don't understand. The word "light" is just a name given to EM radiation in a particular frequency range. You seem to think they are two different things. What do you think the word "light" means?

[edit] ...although sometimes the word "light" is used to name the entire EM spectrum. Usually context will tell you which is being used.
 
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  • #3
hilbert2
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For instance, the fact that good electrical conductors are also often mirror-like reflectors is consistent with light being an electromagnetic phenomenon.
 
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Per the maxwell equations, we know that em waves travel at the velocity of light, but that is not a sufficient condition to say that electromagnetic waves are light. How do we know that electromagnetic waves are light? They could just be something that has the same velocity as light.

Any insight is appreciated.
I would say that if c(the speed of light) is divided by ether the frequency or wavelength, and the result is the one you
did not divide by, then it counts as light.
 
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What I mean is: Maxwell proved that light is an EM wave by showing that v = c via the wave function. But I don't see how showing via the wave function that v = c, automatically means that light is an EM wave.
 
  • #6
russ_watters
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What I mean is: Maxwell proved that light is an EM wave by showing that v = c via the wave function. But I don't see how showing via the wave function that v = c, automatically means that light is an EM wave.
The speed of light had already been measured when Maxwell derived his equations and he merely surmised the speed being the same wasn't a coincidence.
 
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What I mean is: Maxwell proved that light is an EM wave by showing that v = c via the wave function. But I don't see how showing via the wave function that v = c, automatically means that light is an EM wave.
When it meets those criteria, it behaves like a wave, and works with the wave equations.
You could say most of the same things for sound waves, but the "c" is different.
 
  • #8
George Jones
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What I mean is: Maxwell proved that light is an EM wave by showing that v = c via the wave function. But I don't see how showing via the wave function that v = c, automatically means that light is an EM wave.
Yes, you asked a good question.

I think that the experiments of Heinrich Hertz went a long way towards establishing light as electromagnetic waves. (example: above post by @hilbert2 )
 
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I would say that if c(the speed of light) is divided by ether the frequency or wavelength, and the result is the one you
did not divide by, then it counts as light.
What?
 
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Hm. If we see light as a wave, which "wave" is the light that we usually show in diagrams, the B-field or the E-field?
 
  • #11
George Jones
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When it meets those criteria, it behaves like a wave, and works with the wave equations.
You could say most of the same things for sound waves, but the "c" is different.
I think that the experiments of Heinrich Hertz went a long way towards establishing light as electromagnetic waves. (example: above post by @hilbert2 )
See the attached 3-page excerpt from the 600-page book "Modern Physics" by Serway, Moses, and Moyer:
 

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  • #12
ZapperZ
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Hm. If we see light as a wave, which "wave" is the light that we usually show in diagrams, the B-field or the E-field?
For freely-propagating wave in vacuum, it can be either one, because showing one automatically defines the other one.

In a waveguide, this is not so obvious and the geometry of the waveguide determines what the E and B field will look like.

Zz.
 
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See the attached 3-page excerpt from the 600-page book "Modern Physics" by Serway, Moses, and Moyer:
Thanks a lot George! That was nice of you. So now I get it: without Hertz proving that all of the other properties of EM have in fact the same results of "macroscopic" light i.e. interference, refraction, reflection polarisation and etc., then it wouldn't have been enough just to use the wave function and have shown that they both have the same velocity.

Is this why in Optics there is first electromagnetic waves and then geometric/wave optics? Is this to essentially prove the two are one in the same?
 
  • #15
George Jones
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Thanks a lot George! That was nice of you. So now I get it: without Hertz proving that all of the other properties of EM have in fact the same results of "macroscopic" light i.e. interference, refraction, reflection polarisation and etc., then it wouldn't have been enough just to use the wave function and have shown that they both have the same velocity.
Yes. For example, gravitational radiation propagates at c, but gravitational radiation is not light.

Is this why in Optics there is first electromagnetic waves and then geometric/wave optics? Is this to essentially prove the two are one in the same?
I think it is because they have been demonstrated to be the same thing by observations of loads of phenomena, so waves are studied in optics, and the ray approximation of light also is studied.
 
  • #16
DaveC426913
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You can blue-shift light up into UV and higher. Blue shift it enough and it will be detectable as X-rays.

You can also red-shift it down to IR or lower. Red shift it enough and it will be detectable as radio.

In other words, the only difference between light and other parts of the EM spectrum is the frequency.
 
  • #17
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How do we know that electromagnetic waves are light?
Because we have well developed instruments (cameras being one of them), which can only work given this supposition.
Since they do work, the supposition is reasonable.
 
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You can also red-shift it down to IR or lower. Red shift it enough and it will be detectable as radio.
I'm not totally sure we have already proved it experimentally. We have indirect indications (Mossbauer effect, galactic red shift, etc) but do we really proved it in laboratory?

--
lightarrow
 
  • #19
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Per the maxwell equations, we know that em waves travel at the velocity of light, but that is not a sufficient condition to say that electromagnetic waves are light. How do we know that electromagnetic waves are light? They could just be something that has the same velocity as light.
Just to add a bit to the other posts: free electron laser
https://en.m.wikipedia.org/wiki/Free-electron_laser

It generates light by making electrons oscillate. So light must be an electromagnetic wave.
Of course electrons "oscillate" in a sense even when atoms emit light when they undergo a transition from an excited level to the fundamental one (for example) but I don't consider this as a real "prove" that making a charge oscillate at high frequency it generates light, we only have indirect informations that this can be considered as an actual oscillation of the charges, AFAIK.

--
lightarrow
 
  • #20
russ_watters
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I'm not totally sure we have already proved it experimentally. We have indirect indications (Mossbauer effect, galactic red shift, etc) but do we really proved it in laboratory?
It has crossed into the mundane, with every day uses like weather and police radar.
 
  • #21
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It has crossed into the mundane, with every day uses like weather and police radar.
? Maybe you are talking of Doppler effect in general, but I didn't talk about it. I replied to DaveC426913' statement that doppler red shift of light can become radio waves. You knows this fact, I know, everyone knows and we don't need proves of it. But do we have experimental proves of this specific phenomenon? Or of the fact that blue shifting radio waves we can generate light (that is em radiation in the visible spectrum)? I think it's this that the OP wanted to know: which factual proves do we have that light = em waves? About radio waves that can be doppler shifted to light or the other way around , maybe there are laboratory proves but I'm not sure; if someone has more informations about it would be appreciated.
Regards.

--
lightarrow
 
  • #22
russ_watters
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? Maybe you are talking of Doppler effect in general, but I didn't talk about it. I replied to DaveC426913' statement that doppler red shift of light can become radio waves. You knows this fact, I know, everyone knows and we don't need proves of it. But do we have experimental proves of this specific phenomenon?
I suppose not, but so what? Why did you even bring it up if you know there is no need for it?
Or of the fact that blue shifting radio waves we can generate light (that is em radiation in the visible spectrum)? I think it's this that the OP wanted to know: which factual proves do we have that light = em waves?
I can't parse that. It sound circular, like I said in post #2, so I have no idea what evidence you'd accept or why you would connect the question to doppler shift.

I was almost thinking the OP's question was about biology; how do we know what a camera records is what our eyes see. But evidently not.
About radio waves that can be doppler shifted to light or the other way around , maybe there are laboratory proves but I'm not sure...
If I demonstrate in experiments that A=B and B=C, then I have proven that A=C. I don't need a separate experiment demonstrating it.
 
  • #23
Merlin3189
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The OP question is one that had never occurred to me.
If it had, I suppose I'd say that this theory of Maxwell seems to be consistent with experiment, notwithstanding quantum effects, over a long period. That's all that's required of a theory.

On the other side of the coin, if light were not part of the EM spectrum, then there should be some other phenomenon which is EM radiation in that frequency range. Where is it?
 
  • #24
ZapperZ
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The OP question is one that had never occurred to me.
If it had, I suppose I'd say that this theory of Maxwell seems to be consistent with experiment, notwithstanding quantum effects, over a long period. That's all that's required of a theory.

On the other side of the coin, if light were not part of the EM spectrum, then there should be some other phenomenon which is EM radiation in that frequency range. Where is it?
The word "light" here needs to be clarified. Often, it is meant as the EM wave itself, i.e. it isn't referring to just visible light. This is because, for many properties, EM wave is EM wave, no matter the frequency. Singling out just the visible part of this spectrum has no specific purpose.

Back to the OP's question, if one can show that, using a pick-up antenna connected to an oscilloscope, the E-field from, say, a radio wave does oscillate, is this sufficient evidence that light has this wave property?

Zz.
 
  • #25
DaveC426913
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Back to the OP's question, if one can show that, using a pick-up antenna connected to an oscilloscope, the E-field from, say, a radio wave does oscillate, is this sufficient evidence that light has this wave property?
At the risk of speaking for the OP, he asks whether light could just be a lot like EMR (same velocity, same wave nature) while being a qualitatively different animal.
 

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