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• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeJan 9th 2013

at geometry of physics I needed light. So I created a small entry.

• CommentRowNumber2.
• CommentAuthorTobyBartels
• CommentTimeJan 9th 2013

Why restrict to the vacuum?

• CommentRowNumber3.
• CommentAuthorUrs
• CommentTimeJan 9th 2013

True. I have removed that word now.

• CommentRowNumber4.
• CommentAuthorTobyBartels
• CommentTimeJan 9th 2013

Redirect from electromagnetic radiation (and variations).

• CommentRowNumber5.
• CommentAuthorzskoda
• CommentTimeJan 9th 2013
• (edited Jan 9th 2013)

Toby, I think that it would be much more sensible (and traditional) to keep electromagnetic radiation separately from light.

To add some content to my claim (and future separate entry), let me do some discussion. We sometimes say that the light is electromagnetic waves which are a substance, and they often do occur as a result of electromagnetic radiation which is a phenomenon (often related to the consideration of some source) or effect. But of course, sometimes we use the terms interchangeably.

In the standard treatments, the electromagnetic radiation of some body is an electromagnetic field caused by (the dynamics of) some localized body in the limit of a large distance from that body. Thus, the standard books like Jackson have chapters on radiation (meaning) at large distances from sources (what involves theoretical issues like multipole expansions, asymptotics of expressions for fields, retarded potentials and so on), while these are not the beginning chapters on light in the same textbooks. There are also notions like radiative corrections in QFT and so on.

What do you think ? Urs ?

• CommentRowNumber6.
• CommentAuthorUrs
• CommentTimeJan 9th 2013
• (edited Jan 9th 2013)

Certainly light is not the same as electromagnetic radiation. X-rays are not light, for instance.

But as long as we don’t have an entry on electromagnetic radiation… You should create it!

• CommentRowNumber7.
• CommentAuthorzskoda
• CommentTimeJan 9th 2013

I did not mean the difference between light and more general electromagnetic waves but more subtle between the electromagnetic waves and electromagnetic radiation in the sense I explained above.

• CommentRowNumber8.
• CommentAuthorUrs
• CommentTimeJan 9th 2013
• (edited Jan 9th 2013)

In either case.

I think this is entirely standard material that is in the textbooks since the 1860s. We don’t need to have discussion about this, or do we? As soon as somebody writes an entry on electromagnetic radiation, it’s time to remove the above redirect. Be the first!

• CommentRowNumber9.
• CommentAuthorTobyBartels
• CommentTimeJan 9th 2013

I have only seen the distinction that Urs mentions in #6 (which is also hinted at in the article), not the distinction that Zoran is discussing, which is new to me. (This even though I took a course using Jackson; I must not have noticed it there.) I too would like Zoran to write about it. In the meantime, I will also let electromagnetic wave redirect to light.

• CommentRowNumber10.
• CommentAuthorTobyBartels
• CommentTimeJan 9th 2013

(Possibly Zoran or somebody should write radiation first.)

• CommentRowNumber11.
• CommentAuthorTodd_Trimble
• CommentTimeJan 10th 2013

I actually wasn’t aware of the distinction Urs made in #6; I thought physicists were happy to refer to emissions of any frequency across the electromagnetic spectrum as ’light’ (the layman’s ’light’ being humanly visible light). What is the distinction in #6?

• CommentRowNumber12.
• CommentAuthorUrs
• CommentTimeJan 10th 2013
• (edited Jan 10th 2013)

I thought physicists were happy to refer to emissions of any frequency across the electromagnetic spectrum as ’light’

So in astronomy one speaks of “first light” when a telescope receives any kind of radiation for the first time. But I wouldn’t think that otherwise one says “light” for raditation outside the visible, infrared or ultraviolet.

Try it on yourself, how does it sound: does a radio receive light? Does a microwave oven heat with light? Does radium emit light? That all sounds wrong, I’d say.

I think

light = infrared + visible + ultraviolet radiation.

Let’s see what Google thinks.

Here is one hit

We learn about the planets, stars, and galaxies by their light—visible light, and also shorter-wavelength ultraviolet and longer-wavelength infrared light, invisible to the eye but detectable by certain telescopes on Earth and in space—and by the still longer waves of radio energy that they send us.

Elsewhere I see that even only visible light is regarded as light:

Light is the medium of energy by which we perceive our environment through our eyes. It is one form of electromagnetic radiation which includes things like ultraviolet, infrared, and radio waves.

• CommentRowNumber13.
• CommentAuthorUrs
• CommentTimeJan 10th 2013
• (edited Jan 10th 2013)

The Wikipedia entry on “light ” points to one physics textbook that states the photoelectric effect in terms of “light” hitting a surface and then in a footnote says:

The term light includes both visible light and any electromagnetic radiation other than visible light.

But then, this seems to be more like “in this definition ’light’ means…” than “in general ’light’ means… “

In any case, I think what I say in the entry is okay

The waves of the electromagnetic field in electromagnetism are called light (at least those within the frequency range…

but of course anyone should feel free to expand.

(But maybe a pointer to Wikipedia will be enoght to do the $n$Lab purpose justice. This is more about inhabiting links than about spreading information about light. To me.)

• CommentRowNumber14.
• CommentAuthorTodd_Trimble
• CommentTimeJan 10th 2013

Of course laypeople wouldn’t say “radio receives light”, but I thought physicists sometimes used the word more inclusively. Obviously I’m happy to go with what you say is correct.

• CommentRowNumber15.
• CommentAuthorzskoda
• CommentTimeJan 10th 2013

Wikipedia says

Electromagnetic radiation is a particular form of the more general electromagnetic field (EM field), which is produced by moving charges. Electromagnetic radiation is associated with EM fields that are far enough away from the moving charges that produced them, that absorption of the EM radiation no longer affects the behavior of these moving charges. These two types or behaviors of EM field are sometimes referred to as the near and far field. In this language, EMR is merely another name for the far-field.

what is quite alike what I wrote in 5. So I will go on with this.

• CommentRowNumber16.
• CommentAuthorUrs
• CommentTimeJan 10th 2013

That’s good, thanks!

• CommentRowNumber17.
• CommentAuthorzskoda
• CommentTimeJan 10th 2013

electromagnetic radiation, first draft :)

• CommentRowNumber18.
• CommentAuthorUrs
• CommentTimeJan 10th 2013
• (edited Jan 10th 2013)

Thanks, Zoran.

By the way, the above discussion reminds me: while I don’t think that any species of physicists says “light” for “electromagnatic radiation”, it is true that especially astronomers have some ideosyncratic ways of generalizing terms which feels similar. For instance in astronomy they say “metal” for every element that is not hydrogen or helium, as you may have heard. Or “dust” for all matter distribution with a certain equation of state. Our galaxy is just a dust particle in this sense. Which of course makes sense from some perspective.

But in the same vein, I could imagine that an astronomer comes home after a long day at work, brings his kids to bed and then turns off the electromagnetic radiation. ;-)

• CommentRowNumber19.
• CommentAuthorTodd_Trimble
• CommentTimeJan 10th 2013

Winking emoticons aside, I very likely got this idea that physicists use the term “light” more inclusively from Feynman’s book QED (page 13, if you care to look and verify that I’m not just making this up). But once again, you’re the expert Urs, and I defer to your opinion on this matter.

• CommentRowNumber20.
• CommentAuthorUrs
• CommentTimeJan 10th 2013
• (edited Jan 10th 2013)

Hey Todd,

just to be sure: the winking emoticon has nothing to do with your messages here.

Concerning Feynman: I don’t have his book available right now. What does it say there?

• CommentRowNumber21.
• CommentAuthorTodd_Trimble
• CommentTimeJan 10th 2013
• (edited Jan 11th 2013)

See the second paragraph here.

On second thought, let me quote the two paragraphs which gave me the idea that physicists say “light” also to cover radio waves, X-rays, etc.:

• Another possibility, especially if the lecturer is a physicist, is that he uses ordinary words in a funny way. Physicists often use ordinary words such as “work” or “action” or”energy” or even, as you shall see, “light” for some technical purpose. Thus, when I talk about “work” in physics, I don’t mean the same thing as when I talk about “work” on the street. During this lecture I might use one of those words without noticing that it is being used in this unusual way.I’ll try my best to catch myself-that’s my job-but it is an error that is easy to make.

(p. 10)

• When I say “light” in these lectures, I don’t mean simply the light we can see, from red to blue. It turns out that visible light is just a part of a long scale that’s analogous to a musical scale in which there are notes higher than you can hear and other notes lower than you can hear. The scale of light can be described by numbers—called the frequency—and as the numbers get higher, the light goes from red to blue to violet to ultraviolet. We can’t see ultraviolet light, but it can affect photographic plates. Its still light— only the number is different. (We shouldn’t be so provincial: what we can detect directly with our own instrument, the eye, isn’t the only thing in the world!) If we continue simply to change the number, we go out into X-rays, gamma rays, and so on. If we change the number in the other direction, we go from blue to red to infrared (heat) waves, then television waves and radio waves. For me, all of that is “light” I’m going to use just red light for most of my examples, but the theory of quantum electrodynamics extends over the entire range that I have described.

(p. 13)

• CommentRowNumber22.
• CommentAuthorUrs
• CommentTimeJan 11th 2013
• (edited Jan 11th 2013)

Thanks!

So “we shouldn’t be so provincial” ! (I suppress a smiley here.) Maybe you’d enjoy adding those quoted paragraphs to the entry?

• CommentRowNumber23.
• CommentAuthorTodd_Trimble
• CommentTimeJan 11th 2013

Okay, I added a terminology section to light, but those who speak the language more fluently should please feel free to edit.

• CommentRowNumber24.
• CommentAuthorTobyBartels
• CommentTimeJan 11th 2013

Is Zoran happy with this?

• CommentRowNumber25.
• CommentAuthorUrs
• CommentTimeJan 11th 2013

By the way, you have all heard of the bike-shed effect, I suppose? :-)

• CommentRowNumber26.
• CommentAuthorzskoda
• CommentTimeJan 11th 2013

24: I just wanted to have electromagnetic radiatiopn distinct from waves and I did the entry for that. So this is OK. I said nothing about other things anyway.

• CommentRowNumber27.
• CommentAuthorTodd_Trimble
• CommentTimeJan 11th 2013
• (edited Jan 11th 2013)

Re #25: actually, I hadn’t, but I gather it has something to do with wasting time on minutiae. Why do you ask?

I thought your #22 was intended as encouragement to me to add something to the entry, but it’s possible I misunderstood your intention. Basically, all I asked in the beginning was clarification on what the distinction was in #6. Mostly just for my own education.

I’m done with this for now; please feel free to modify the entry as you wish. (And thanks, Zoran.)

• CommentRowNumber28.
• CommentAuthorTobyBartels
• CommentTimeJan 11th 2013

Yes, I know the bike shed effect, but if there is a difference between waves and radiation, and if light is (in general) one of these and not (necessarily) the other, then I think that it's important to straighten that out.

I remain confused, because Zoran said first that light is waves and not (necessarily) radiation, but now light says that light is radiation (and also says that it's waves). Zoran did change ‘light is elctromagnetic radiation’ to ‘light is appearing as elctromagnetic radiation’, but below under Terminology it flat out says that light is radiation.

• CommentRowNumber29.
• CommentAuthorTodd_Trimble
• CommentTimeJan 11th 2013

Maybe we need entries: wave and radiation. I didn’t really understand what is trying to be conveyed by “light is appearing as electromagnetic radiation”. The terminology section is what I wrote myself, without thinking very hard about it.

• CommentRowNumber30.
• CommentAuthorTobyBartels
• CommentTimeJan 12th 2013

We do have wave (also oscillation), but not radiation.

I find the state of wave rather wanting (which is not to say that I know how to fix it!). In particular, one of the key properties of a wave (as opposed to an oscillation, which seems more general) is claimed to be a medium, but that doesn't seem to be necessary for light; the idea that light may be a wave in no medium was a big deal about a hundred years ago. (Also, Isn't there a background-indpendent notion of a wave in spacetime that we can describe as more fundamental than the definition there? Maybe not!)

• CommentRowNumber31.
• CommentAuthorzskoda
• CommentTimeJan 14th 2013
• (edited Jan 14th 2013)

The difference is between structure and property. Do we consider the em field the property of a state of an “empty space” or this is an additional structure, so that the space is in fact not empty ? In any case, the vacuum which can carry a bundle with gauge fields may be considered a medium for waves of the gauge fields, a medium in modern sense. Not a medium in the sense of bunch of particles…what would fit 18th century view of a medium.

Zoran said first that light is waves and not (necessarily) radiation

Well, I think it is OK both as I said and the opposite. When say it is not necessarily a radiation that is a radiation in a narrow sense, i.e. a radiation field of something. In a wide sense all energy of fields passing through a medium and not the energy of a moving medium itself is considered a radiation, in that sense all em waves could be viewed as radiation. There are obviously subtleties of usage of electromagnetic radiation.

• CommentRowNumber32.
• CommentAuthorUrs
• CommentTimeJan 15th 2013

Why do you ask?

Because the effect is quite in effect in our little community, and it might help to be aware of it.

• CommentRowNumber33.
• CommentAuthorTodd_Trimble
• CommentTimeJan 15th 2013

Dear Urs: I don’t regard trying to get straight answers to honest well-intentioned questions a waste of time. (Actually, I can’t even tell who or what your comment in #25 is directed towards, exactly, but it sure came across as snarky.) It looks like Toby wanted to get certain things straightened out as well. If you’d rather not spend time on this, then please spend your time on something you consider more productive.