Talk:Electromagnetic spectrum

EMR acronym

Electromagnetic radiation is introduced at the intro to article. In the body of article EMR is used for the concept on several places. When a novice reader is focused on a part of the body text and only skims the surroundings, EMR acronym's meaning might not be obvious, and search within page would not quickly bring full text to her or his attention.

I propose the change at first use to electromagnetic radiation (EMR)' in intro; if nobody opposes I'll do it in a day or two. Marjan Tomki SI (talk) 14:05, 23 December 2022 (UTC)

Go ahead. In fact, if you can repeat the full name and acronym in the body in a few places, especially deep down in the article. Constant314 (talk) 16:55, 23 December 2022 (UTC)

What is the first sentence trying to say?

To this mathematician, the meaning of the phrase "the variation in the intensity of electromagnetic waves with respect to their frequency or wavelength of oscillation" (as well as the earlier versions "range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies" and "ranging over a domain of frequencies and their respective wavelengths and photon energies") is close to incomprehensible. The version that I wrote made sense to me (well yes of course); the revert edit summary does not. (I also think "The electromagnetic spectrum is the spectrum of electromagnetic radiation" is so obviously tautological as to be pointless, but at least I understand what it says.) Would value input from other editors of this page. --JBL (talk) 17:43, 6 December 2023 (UTC)

I don't like the current lead either, the term "variation in intensity" is misleading. It is unsourced and doesn't appear to be supported by any sources I've seen. Agree that the phrase "the spectrum of electromagnetic radiation" is tautological and unnecessary. I support your version or the first of the two you quote above (I don't like the word "domain" in the second). Here are some other definitions:

• "the entire distribution of electromagnetic radiation according to frequency or wavelength." Encyclopedia Britannica online.
• "the entire range of wavelengths of all known electromagnetic radiations." European Commission glossary
• "the range of all types of electromagnetic radiation." NASA Introduction to Astronomy page Too vague.

--Chetvorno^TALK 20:46, 6 December 2023 (UTC)

Thanks for your feedback, Chetvorno. If I understand correctly, the point is that the electromagnetic spectrum means the full range of electromagnetic radiation; and that this range may be expressed in various ways (in terms of frequencies, or wavelengths, or ...). To be honest I actually like the NASA version, which sets the question of what language do you express the spectrum in (frequency, wavelength, ...) aside as a separate point (and this is also what I was trying to achieve in my edit). Yet another possibility would be to write the lead paragraph without the structure "The [title of article] is". For now, I'm going to restore my previous edit -- but of course if it's still disputed I would be happy to work on alternatives. --JBL (talk) 18:08, 8 December 2023 (UTC)

I kind of like the NASA version too. It's on the vague side, but the following sentences would provide the specifics. XOR'easter (talk) 20:23, 8 December 2023 (UTC)

I feel pretty strongly that the terms frequency, wavelength or both need to be in the lead sentence. The word 'range' by itself is ambiguous. The introduction is for general readers, including kids, high school dropouts and ballet dancers, who may have never had any science education at all. Their reaction to The electromagnetic spectrum is the range of all types of electromagnetic radiation is likely to be "Range . . . oh, that means the distance it can go." Then efforts in the following sentences to define the term as frequency or wavelength will confuse them. --Chetvorno^TALK 23:42, 8 December 2023 (UTC)

Maybe. I'm not sure that holding off the details to the second sentence would be more confusing than including them in the first. If a reader is thinking that range means "how far it can go", surely they'd trip up when they hit "frequency" or "wavelength" whether those are included in the first sentence or deferred to the next. I'd be inclined to guess that a shorter first sentence makes for easier reading, but that's just a feeling. XOR'easter (talk) 17:37, 9 December 2023 (UTC)

Electromagnetic spectrum is part of Spectrum (physics), which is defined as "plot of light intensity or power as a function of frequency or wavelength, also known as a spectral density plot." The spectrum is not just an interval (much less its two limiting values, inferior and superior, or the corresponding width or range) of frequency or wavelengths. After all, the full frequency band of EM waves is [0,∞], but that's just the domain of the spectrum, which is actually a function. The function's value or image is normally the EM wave's spectral power or intensity, somtimes its square-root ("root-power", magnitude, or amplitude). It can be understood as the result of Fourier analysis applied to sums of myriad monochromatic radiation waves; is has physical implementations, such as dispersive prisms. fgnievinski (talk) 05:12, 9 December 2023 (UTC)

which is defined as No I don't think so -- that is one definition, but it's not the only (and outside of specialist contexts at least not the primary) definition. I mean just look at the lead sections of Spectrum (physical sciences) and Spectrum. --JBL (talk) 17:22, 9 December 2023 (UTC)

The "electromagnetic spectrum" is the domain. A function on that domain would get a different, more specific name: "blackbody spectrum", an "absorption spectrum", etc.

See, for example, the typical usage in a college astronomy textbook: Objects in the universe send out an enormous range of electromagnetic radiation. Scientists call this range the electromagnetic spectrum, which they have divided into a number of categories. Or a chemistry textbook: This figure also shows the electromagnetic spectrum, the range of all types of electromagnetic radiation. And a typical phrasing from later in the book: Compare the two types of emission spectra: continuous spectrum of white light (top) and the line spectra of the light from excited sodium, hydrogen, calcium, and mercury atoms. Or, if you want a straight-up physics reference, Table 9.1 in Griffiths' Introduction to Electrodynamics is "The Electromagnetic Spectrum", and it lists frequencies, their corresponding wavelengths, and names where applicable: intervals, but not functions on those intervals. Problem 9.23 reads, If you take the model in Ex. 4.1 at face value, what natural frequency do you get? Put in the actual numbers. Where in the electromagnetic spectrum does this lie, assuming the radius of the atom is 0.5 Å? The term "electromagnetic spectrum" is being used to refer to the interval, not any function on it. The Feynman Lectures on Physics use the term as follows: The fact that we can see in a particular frequency range makes that part of the electromagnetic spectrum no more impressive than the other parts from a physicist’s standpoint, but from a human standpoint, of course, it is more interesting. And later: This is the first of a number of chapters on the subject of electromagnetic radiation. Light, with which we see, is only one small part of a vast spectrum of the same kind of thing, the various parts of this spectrum being distinguished by different values of a certain quantity which varies. XOR'easter (talk) 17:26, 9 December 2023 (UTC)

fgnievinski: agree with JBL and XOR'easter. The electromagnetic spectrum of some specific object, such as "the electromagnetic spectrum of the sun" would be the intensity plot vs freq or wavelength as you say, but "the electromagnetic spectrum" alone refers to the different types of radiation, or the frequency or wavelength limits. --Chetvorno^TALK 22:42, 9 December 2023 (UTC)

Well, it's both. The two meanings are well captured in the Electropedia. (Context: that's an online glossary organized by the International Electrotechnical Commission, based on the International System of Quantities and other ISO standards and similar authoritative sources.) See:

spectrum: representation of a real or complex quantity as a function of frequency.[1]

Note 1 to entry: Other quantities related to frequency are often used as variables, e.g. wavelength in vacuum or angular frequency.

Note 2 to entry: The word "spectrum" is also used to denote the frequency band where some phenomenon occurs, e.g. acoustic spectrum, visible spectrum.

Then are subclasses of spectrum (main connotation):

power spectrum: the distribution as a function of frequency of the square of the amplitudes of the spectral components of a signal or noise.[2]

Note – The spectrum of an electromagnetic radiation, defined in chapter 845, can be considered as a power spectrum.

amplitude spectrum: the distribution as a function of frequency of the amplitude of the spectral components of a signal or noise,[3]

phase spectrum: the distribution as a function of frequency of the initial phases of the spectral components of a signal or noise.[4]

fgnievinski (talk) 04:22, 12 December 2023 (UTC)

The "spectrum of an electromagnetic radiation" is not "the electromagnetic spectrum". It's the spectrum of a particular radiation source. The actually relevant part of the source you quote is that it agrees "spectrum" can mean "the frequency band where some phenomenon occurs" and uses "visible spectrum" as an example. The electromagnetic spectrum is a superset of the visible spectrum; i.e., it's a bigger frequency band. Not any function defined on that band. XOR'easter (talk) 15:53, 15 December 2023 (UTC)

Physics articles should focus on the physical phenomenon, not on the mathematics we use to describe it. The electromagnetic spectrum is not a function—that is a mathematical concept, not a physical one. It's not the domain either. I would rephrase what JBL wrote above this way: "The electromagnetic spectrum is the full range of electromagnetic radiation, from the lowest-frequency radio waves to the highest-frequency gamma rays." That gets us in the mindset of physics from the start and focuses on the topic itself, leaving the distraction of frequency vs wavelength vs photon energy for a subsequent sentence.--Srleffler (talk) 04:51, 12 December 2023 (UTC)

My feeling is this lead sentence is not as good as the previously suggested ones. It uses the word 'frequency' without stating the relation of that to the subject; that the spectrum is a range of frequencies, and it uses 'radio waves' and 'gamma rays' as if the reader already knows what those are, that they are frequency bands in the electromagnetic spectrum. For nontechnical readers we should just tell them these things: "The electromagnetic spectrum is the distribution of electromagnetic radiation according to frequency or wavelength" --Chetvorno^TALK 07:42, 12 December 2023 (UTC)

I like @Srleffler version, but I added "frequencies" per @Chetvorno's comment.

We can't introduce the entirety of electromagnetic theory in one sentence, so the non-technical reader will have to keep reading or decide they've had enough. Radio waves are commonly known; we could use "X-rays and gamma rays" for the high end since X-rays are also commonly known. Maybe "radio waves and microwaves" for the low end to be parallel. Johnjbarton (talk) 16:55, 12 December 2023 (UTC)

The new version doesn't indicate that the list of types of electromagnetic waves is ordered by frequency, nor whether it proceeds from low to high or high to low frequency. Second, electromagnetic waves at the high energy end of the spectrum are usually not measured by frequency but by wavelength, so wavelength should be mentioned. This is getting worse and worse. Per BRD changed to my alternative lead which fixes these problems. --Chetvorno^TALK 17:42, 12 December 2023 (UTC)

The EM spectrum is not the full "range" or multitude of EM radiation. Actually, it's only the variation in a few key properties of EM radiation: frequency (or wavelength) and power (or amplitude), sometimes also phase offset. There are other properties of EM radiation which are not normally considered in the EM spectrum, such as polarization (or direction of oscillation), direction of arrival (or the corresponding angles), degree of coherence, etc. We should stick to the sources, especially secondary and tertiary sources, which give two clear meanings: EM spectral density and EM frequency domain. Admittedly, the latter is only useful when partitioned in frequency bands, otherwise the spectrum of any wave phenomena (EM, acoustic, water, etc.) is the trivial half-line. fgnievinski (talk) 03:50, 13 December 2023 (UTC)

fgnievinski: In case you didn't see it, I listed definitions above from 3 secondary or tertiary sources that say the electromagnetic spectrum is the range of frequencies or wavelengths. Here are some more:

• "The electromagnetic spectrum categorizes electromagnetic waves according to frequency and wavelength." Tammaro, College Physics, p.677
• "Electromagnetic spectrum: the range of wavelengths over which electromagnetic radiation extends." Jonathan Law, A Dictionary of Science
• "Electromagnetic spectrum: the range of electromagnetic waves listed in order of increasing wavelength and named in segments according to their mode of generation" Chappie, Dictionary of Physics, p.74
• "It is the frequency of an EM wave that determines its characteristics...The electromagnetic spectrum describes the types of electromagnetic waves observed at the specified frequencies and wavelengths" Fullerton, Guide to Regents Physics Essentials, p.219

--Chetvorno^TALK 20:50, 13 December 2023 (UTC)

Except for the second source, none of them imply EM spectrum equals a frequency or wavelength range (meaning interval, not width). Instead, they state frequency is the independent variable or domain over which EM radiation varies. EM spectrum is implied in the quoted sources as the variety of EM radiation or EM waves as a function of frequency or wavelength. That's still a qualitative notion of EM spectrum, because EM waves have many characteristics; whenever one needs to quantity the spectrum, a given property needs to be selected, most commonly the EM power and its variation with respect to frequency or wavelength. fgnievinski (talk) 03:39, 14 December 2023 (UTC)

"Spectrum" has multiple definitions. The topic of this article is or should be the physical but qualitative definition: it's the range of electromagnetic radiation, organized by wavelength or frequency. The topic is not simply EM power as a function of wavelength. That is a distinct definition of the term, covered at spectral power. The topic here is the EM radiation itself, not a single property of it. The definitions above by Tammaro, Chappie, and Fullerton agree with this. The definition by Law does not. The topic is not the range, it's not the power, it's the waves themselves.--Srleffler (talk) 08:06, 14 December 2023 (UTC)

Exactly. That’s what I was trying to say. Agree 100%. Well put--Chetvorno^TALK 17:56, 14 December 2023 (UTC)

Regarding We should stick to the sources, especially secondary and tertiary sources, which give two clear meanings: EM spectral density and EM frequency domain. Except that they don't. The sources give one clear meaning for "electromagnetic spectrum" in its own right, and other clear meanings for other uses of the word "spectrum". XOR'easter (talk) 15:44, 15 December 2023 (UTC)

Reversed numbers?

Why didn't anyone notice the MIR and FIR values for Photon eV is reversed. Wrong numbers in the table. 65.36.54.181 (talk) 13:01, 19 December 2023 (UTC)

They do not look reversed to me.--Srleffler (talk) 06:07, 20 December 2023 (UTC)

Use frequency in discussions of energy.

I just stumbled upon the UV section: lots of "short wavelength" discussion. I encourage editors to avoid using "wavelength" in any sentence here about energy. Energy is proportional to frequency. Using the inversely proportional wavelength is unnecessarily confusing and physically not helpful. Johnjbarton (talk) 21:30, 27 January 2024 (UTC)