How do we know electromagnetic waves are light?

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.

russ_watters
Mentor
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?

 ...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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hilbert2
Gold Member
For instance, the fact that good electrical conductors are also often mirror-like reflectors is consistent with light being an electromagnetic phenomenon.

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.

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.

russ_watters
Mentor
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.

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.

George Jones
Staff Emeritus
Gold Member
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 )

PeroK and mk9898
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?

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?

George Jones
Staff Emeritus
Gold Member
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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• Hertz and Light1.pdf
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Merlin3189 and mk9898
ZapperZ
Staff Emeritus
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.

mk9898
What?
C/wavelength=frequency, C/frequency=Wavelength

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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?

George Jones
Staff Emeritus
Gold Member
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.

DaveC426913
Gold Member
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.

russ_watters and Merlin3189
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.

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?

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lightarrow

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.

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lightarrow

russ_watters
Mentor
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.

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.

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lightarrow

russ_watters
Mentor
? 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.

Merlin3189
Homework Helper
Gold Member
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?

davenn and russ_watters
ZapperZ
Staff Emeritus
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.

russ_watters
DaveC426913
Gold Member
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.

DaveC426913
Gold Member
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.
Indeed.

Light - being some wave-like propagation that moves at c but not being EMR - that would raise more questions than it answers.
We would be multiplying entities unnecessarily.

russ_watters
Lightarrow wrote:
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,
Ok, I simply liked to know this, nothing else.
Lightarrow:
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.
No, probably I expressed myself bad, I simply liked to know about experimental evidence relating visible light <- doppler shift -> radio waves.
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.
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.
And in this case what are A, B and C?
Regards.

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lightarrow

Drakkith
Staff Emeritus
No, probably I expressed myself bad, I simply liked to know about experimental evidence relating visible light <- doppler shift -> radio waves.

We use the doppler effect in doppler cooling (a subset of laser cooling). The underlying math is exactly the same for laser light, radio waves, and all other types of EM radiation.

We also use doppler shift to find binary stars and exoplanets in a technique called doppler spectroscopy.

Staff Emeritus
2021 Award
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.

But isn't that like asking whether the Iliad was written by Homer or by a different blind poet with the same name?

russ_watters
russ_watters
Mentor
And in this case what are A, B and C?
Regards.
Doppler shift from/to different frequencies.

DaveC426913
Gold Member
But isn't that like asking whether the Iliad was written by Homer or by a different blind poet with the same name?
Indeed it is. The OP is needlessly multiplying ancient blind poets.

DaveC426913
Gold Member
I'd like to suggest to the OP that we start with the working assumption that:
1. if it walks like one of an unbounded spectrum of other ducks, and
2. talks like one of unbounded spectrum of other ducks, then
3. it's certainly easier to posit that it's one of many such ducks, and it would save a lot of time and confusion if
4. the OP expounds upon their reasoning as to why they think it might not be a duck.

russ_watters
Mentor
Per a previous post of mine, the OP can also be read as a clumsily-written history question:
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.
If that's what it is about, the clumsy wording is in part caused by not understanding the history, which makes it difficult to ask the question well.

Consider if radio waves had been discovered before the EM radiation had been conceived of and before light's speed had been measured instead of the other way around. Then scientists would have had radio waves and light, neither of which would have had an explanation and there would't have been any reason to believe they were connected phenomena.

Instead, light's speed was known and EM radiation, including radio, was predicted at a speed equal to that of light. So the connection was made immediately and there never was any time when both were known of when it wasn't believed that "light" was a form of "EM radiation".

But isn't that like asking whether the Iliad was written by Homer or by a different blind poet with the same name?
I sincerely don't understand answers like this. The question is simple: we have visible light, the one we know since we are born, we know this concept long before having heard of em radiation for the first time. Then we study at school em radiations and we read in a high school textbook that light is "em radiation" . The question is: "how do we know it? ". Nothing else than this.
Regards.

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lightarrow

We use the doppler effect in doppler cooling (a subset of laser cooling). The underlying math is exactly the same for laser light, radio waves, and all other types of EM radiation.
We also use doppler shift to find binary stars and exoplanets in a technique called doppler spectroscopy.
And in these phenomena visible light is red-shifted to radio waves or radio waves are blue-shifted to visible light, as I was asking?

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lightarrow