Talk:Electromagnetic radiation

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Implement a structure following Wikipedia:WikiProject Science guidelines: Introduction accessible to the general public and citing related topics What is electromagnetic radiation? Types/sources of electromagnetic radiation Properties of electromagnetic radiation (power, energy, momentum, polarization) Re-read first paragraph under heading Properties History History of understanding safety around radioactivity edit the first sentence for vandalism
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1 Comments
2 Ratio of ?
3 Continuum of Frequencies in the Spectrum?
4 Merge Light and Electromagnetic Radiation??
5 Electromagnetic radiation
6 Gamma rays
7 Cause of electromagnetic spectrums
8 Similar to Electromagnetic spectrum
9 X-rays vs gamma rays
10 Moving charge
11 wavelets
12 Electromagnet Shield
13 Klystrons and ionising radiation
14 Natural and man made
15 Derivation
16 Where is the energy stored?
17 German version has a very nice pic
18 Electromagnetic waves
19 Direction of propagation
20 Induction vs Hertzian Wave
21 electromagnetic radiation and light
22 The Null Effect of magnetic or electric fields on EM radiation/light
23 All of the EM spectrum is light?
24 Explanation for a Grunt Lost in Space
25 Misleading mention of AM and FM in diagram
26 Merge
27 Error on diagram
28 Electrosmog
29 Accelerated electrons
30 Direction of propogation
31 Effect of Rare Earth Magnet on EM Radiation: None !!!
32 Ambient electromagnetic radiation
33 A clarification
34 Serious Diagram Error
- 34.1 Several Errors
- 34.2 COMMENT - Disagree with the comment and response to comment
35 Serious phase errors!
36 Old Revision Reverted (Opening Paragraph)
37 "Eighty-one octaves"
38 Properties Diagram Is For t = constant?
39 Informal RfC for 'Speed of light' article
40 Why is this article called Electromagnetic radiation, not Electromagnetic wave?
41 Merge with light
42 EM wave quantized
43 Gravity
44 "EM radiation carries energy and momentum that may be imparted to matter with which it interacts."
45 Question
46 Mass Question
47 Experiments Section or New Page
48 Electric vs Magnetic Phasing

[edit] Comments

Answer to Robert van Hoff: Dear Sir I think your answeres are in our references below :Dr Bo Thelin,btn602003@yahoo.se

A new spectral line intensity formula for optical emission spectroscopy was presented by Dr:s Bo Thelin (experimental physicist) and Sten Yngström (theoretical physicist) at the beginning of the 1980:s. This formula has shown very good agreement between experiments (Ref 1) and the new theory (Ref 2). These references are summaries of earlier papers.

I = K \ left (frac { v^2 }{ c^2 }\ right ) \ e^left (\ frac { -J}{ k \ T \ right )} \ \ left (\ e^left ( \ frac { h \ v }{ k \ T \right - \ 1)^-1

K includes transition rates and element concentrations I= spectral line intensity, v= frequency , J=ionization energy and T=temperature

Many independent experimental methods strongly support the new formula, while the standard intensity formula with the Boltzmann term (upper energy level), deviates very much from experiments. Ref 1 Yngström,S. and Thelin,B. Applied Spectroscopy, 44, 1566, (1990) Ref 2 Yngström,S. International Journal of Theoretical Physics,Vol.33, No 7,(1994) —Preceding unsigned comment added by 79.138.180.147 (talk) 13:50, 11 March 2008 (UTC)

[edit] Ratio of $\frac{E_0}{H_0}$ ?

The article says at the end $E 0 = c 0 B 0$ while the book "Fundamentals of Photonics" says $\frac{E_0}{B_0}=c*\mu=\sqrt{\frac{\mu}{\epsilon}}$ So is there something wrong or did I miss a point?

[edit] Continuum of Frequencies in the Spectrum?

I cannot find a reference on whether frequencies of electromagnetic radition constitute a perfectly smooth continuum with all possible frequencies represented, or are frequencies 'clustered' with some frequencies in the spectrum completely absent?--Robert van der Hoff 03:18, 3 May 2007 (UTC)

Frequencies are continuous, in general, for example in a black-body spectrum. For simple systems like isolated atoms, the spectrum is discrete. Dicklyon 03:44, 3 May 2007 (UTC)

Thank you! While the spectrum is continuous, it is not infite. What factors determine its upper and lower limit? I can't find a reference for simple minds like mine explaining why EM waves are self-propagating, and why at the speed of light and not any other speed, or why they just don't stand still? --Robert van der Hoff 03:49, 3 May 2007 (UTC)

See black body for the equation that determines the spectrum of black-body light source. It is nonzero at all energies (or frequencies, or wavelengths), but at high enough frequencies (short enough wavelenghts) is an exponentially decreasing function of energy, so gets pretty close to zero, at energy proportional to absolute temperature. The propagation follows from Maxwell's equation; the speed of propagation is determined by a couple of the constants (the electric and magnetic constants of free space, I forget what they're called). That's how the speed of light and the properties known to electrical theorists turned out to be related through Maxwell's equations. It's not so easy to explain in layman's terms, but I'm sure you can find good descriptions some place, or maybe someone else can try to describe it better here. Dicklyon 01:29, 4 May 2007 (UTC)

Your explanation is much appreciated--Robert van der Hoff 09:29, 4 May 2007 (UTC)

[edit] Merge Light and Electromagnetic Radiation??

Merging the topic electromagnetic radiation with the topic light would make no sense since many forms of electromagnetic radiation are not IR, UV or visible light. Most people would consider light to be a seperate though related topic which should be listed and accessed seperately.

Electromagnetic radiation within a specific frequency range happens to be visible to the human eye, and is called light. —Preceding unsigned comment added by 84.86.91.251 (talk) 13:20, 26 April 2008 (UTC)

[edit] Electromagnetic radiation

The term electromagnetic radiation is also used as a synonym for electromagnetic waves in general, even if they are not radiating or travelling in free space. This sense includes, for example, light travelling through an optical fiber, and electrical energy travelling within a coaxial cable.

I removed the above paragraph as I find it misleading. Electromagnetic waves are always "radiating", to radiate is to move in "rays" and that is how electromagnetic waves propagate. Also the sentence about electrical energy is just plain wrong, it is not correct to describe the movement of electrons in a cable of any type as an electromagnetic wave. --Jpowell 23:25, 18 June 2006 (UTC)

[edit] Gamma rays

How it is possible that some gamma rays have longer wave length than some x-rays? They would be called x-rays then, wouldn't they? Or is there some definition of gamma rays which does not refer to wavelength? --AxelBoldt

My understanding is that at least originally, gamma-ray was the name given to the photons generated from nuclear decay. X-rays on the other hand were generated by electronic transitions involving highly energetic inner electrons. Therefore the distinction between gamma-ray and x-ray is related to the radiation source rather than the radiation wavelength. Generally, nuclear transitions are much more energetic than electronic transitions, so most gamm-rays are more energetic than x-rays. However, there are a few low-energy nuclear transitions (eg. the 14.4 keV nuclear transition of Fe-57) that produce gamma-rays that are less energetic than some of the higher energy x-rays.

--Matt Stoker

The 'conflict' with some gamma rays having longer wave length than some X-rays arises becauses we use the terms (gamma ray and X-ray) for both i) certain parts of the electromagnetic radiation spectrum and ii) electromagnetic radiation from certain processes. --Css

Maybe that should be clarified in the article? —Preceding unsigned comment added by 128.205.204.56 (talk) 15:58, 2 April 2009 (UTC)

[edit] Cause of electromagnetic spectrums

The recently added section titled "What causes electromagnetic spectrums" would probably be more appropriate on a page about Spectroscopy. In my opinion the electromagnetic radiation page should be constrained more to a discussion of the properties of the radiation itself, perhaps with references to spectroscopy and other uses of the radiation. Also, the new section needs some work, since emission and absorption of quanta are not only associated with electronic transitions, but are also associated with rotational, vibrational, and nuclear transitions. Also the section titled "Temperature" has some problems, since the continuous spectrum is not due to doppler broadening of atomic emissions, but is more likely due to vibrational emissions. --Matt Stoker

It have moved my section to spectroscopy - I agree this seems more relevant. I was also dubious of the doppler effect being the cause of continuous spectrum but it was the only cause I could find. Thanks for advice. -- sodium

[edit] Similar to Electromagnetic spectrum

This article is very similiar in topic to Electromagnetic spectrum -- The Anome

should these be mereged? JDR

I think there's a place for both, as this one could address radiation in general - not just the frequency ranges of radiation. It would need a lot of work though. --Laura Scudder 22:13, 31 Mar 2005 (UTC)

This article is referenced about 360 times, so there is a clear demand for a catchall article on electromagnetic radiation, but I find the current version unsatisfactory. With this in mind I created a To do list that's mostly about article structure to get it going. Laura Scudder 00:12, 1 Apr 2005 (UTC)

[edit] X-rays vs gamma rays

By definition the difference between X-rays and gamma rays is that X-rays are produced by electrons releasing energy in the form of photons when they change energy levels while gamma rays are released by the nucleus as part of the process of radioactive decay. X-rays can have ridiculously high eV, even overlapping the wavelengths of more normal gamma rays but are still characterized as X-rays if they originate from electrons. -- Alex.tan 07:11, 16 Sep 2003 (UTC)

[edit] Moving charge

Thought I'd address the accelerating versus moving charge. Not all moving charge creates radiation. For instance, an infinite wire carrying constant current is an arrangement with moving charges. However, there is only a constant magnetic field - no electric field - so no power is radiated. Therefore, moving charges do not always create radiation. Accelerating charges do always create radiation. I think the author was explicitly thinking of Bremsstrahlung and synchrotron radiation. -- Laura Scudder 22:13, 31 Mar 2005 (UTC)

[edit] wavelets

The "wavelet" article link referred in 1.1 points talks about the algorithm, not the physics aception. I don't know what should be done about this since it's the only article on wavelets. I guess a disambiguation page on wavelets could be added, and then a stub about "wavelets" in the sense needed here. I'm not really sure about how to do this since I haven't edited the Wikipedia in a while, so i'm just posting it here.

Yes, this is unfortunate. I suggest that the present wavelet article should be renamed wavelet analysis, and wavelet should be used for the principle in physics. --Heron 16:24, 3 September 2005 (UTC)

"Wavelets" must be removed here. A quick google search on "wavelets" will confirm for anyone what that word means nowadays, and what it will likely continue to mean for a long time to come.

[edit] Electromagnet Shield

I live near high voltage electrical cable. How can I make something simple - not to expensive - to shield my monitor from electromagnet field? -Hace--

Turn your room into a Faraday Cage. (CHF 05:47, 28 October 2005 (UTC))

A sheet of ferrous metal (like the metal used for repairing car bodies) attached to a wall or floor between the monitor and the source of em interference may reduce the effects on the monitor. Due regard must be given to not blocking ventilation of the monitor, not shorting out electrical conductors, avoiding sharp edges etc etc. Images on a monitors may do a "hula dance" in the presence of a 60 Hz field, as when a transformer or AC power cable is nearby. A nearby DC magnetic field, as from subway power cables,or a bank of storage batteries may cause the image to shift laterally.Edison 21:56, 6 June 2006 (UTC)

[edit] Klystrons and ionising radiation

Just a stupid question. I didnt realise that klystrons gave off ionising radiation. depends on the voltage on the anode I suppose. Does any one have any details?. ie what sort of radiation is given off: X rays or what?--Light current 18:56, 11 September 2005 (UTC)

[edit] Natural and man made

Natural and man made should be distinguished soon. The term radiation, is commonly used for this type of energy, although it has a broader meaning, Also called electromagnetic energy or simply radiation. Scott 14:05, 16 October 2005 (UTC)

[edit] Derivation

I added a derivation from Maxwell's equations. I feel this is one of the most important derivations in all of physics. (CHF 06:38, 28 October 2005 (UTC))

Excellent work! This gives the article more authority. --Heron 16:04, 28 October 2005 (UTC)

Broken code (Internet Explorer 6 and Firefox 1.5.0.2); repetitive "Failed to parse (Can't write to or create math output directory)" notifications with varying parameters. Not sure if anyone else sees it, I barely know Wiki so its impossible for me to fix it unfortunately (if its a global bug.) --Phopojijo 17:56, 19 April 2006.

I changed a minus sign in equation (6), small error

[edit] Where is the energy stored?

Since the electric wave and the magnetic wave are in phase, they both become zero at the same time. At these times, where is the energy of the wave stored?--Light current 03:00, 26 December 2005 (UTC)

Why, then the energy is in places where the electric and magnetic field are not zero. There's always the same number of peaks and nodes in the wave at all times. — Laura Scudder ☎ 17:31, 26 December 2005 (UTC)

Yes, good answer! I suppose if the wave is travelling (which it is) then the wave packet always contains energy also travelling. I suppose I must have been thinking of a zero length wave packet which of course can contain no energy--Light current 19:31, 26 December 2005 (UTC)

Electromagnetic energy travelling at the speed of light in free space is actually not stored energy - it is radiating energy. Energy is stored when it is confined with no velocity. - Marlowgs 13:59, 21 March 2006 (UTC)

In general I don't think that the electric and magnetic fields are ever zero at the same time. The the relationship between the two is that the magnetic field is the derivative of the electrive field with respect to time (and vice versa according to Maxwell's Equations?) which means that they are never in phase: when the electric field is at its peak the magnetic field is at zero and vice versa. Therefore one is always nonzero and "storing" the energy. Someone please correct me if I'm wrong on this.

You are wrong on this, I'm afraid. Take a look at Electromagnetic wave equation#Solutions to the homogeneous electromagnetic wave equation, particularly at the sinusoidal steady-state case, and you will see that E and H are always in phase. --Heron 20:44, 12 April 2006 (UTC)

I'm less sure about this. See http://www.play-hookey.com/optics/transverse_electromagnetic_wave.html for an example discussion where the author believes the electric and magnetic fields to be out of phase with one another. The link Heron provided above seems to be solving the B and E form of the equations separately, which gives the same answer since they are essentially identical equations (see the beginning of that article). But the point is that each one is independently a solution. You'd need to use Maxwell's eqn to derive the value of the other field. Grj23 (talk) 20:38, 29 April 2009 (UTC)

In fact it's pretty clear, I think. If

$\mathbf{E}(\mathbf{r},t) = \mathbf{E}(\mathbf{r})\cos(\omega t)$ then from Maxwell's equation: $\nabla\times(\mathbf{E}(\mathbf{r})\cos(\omega t))=-\partial\mathbf{B}/\partial t$ and so $\mathbf{B} = -(\nabla\times\mathbf{E}(\mathbf{r})/\omega)\sin(\omega t)$ Similarly if you start with $\mathbf{B} = -\mathbf{B}(\mathbf{r})\sin(\omega t)$ then $-(\nabla\times\mathbf{B}(\mathbf{r}))\sin(\omega t) = \mu_0\epsilon_0\partial\mathbf{E}/\partial t$ and so $\mathbf{E} = (\nabla\times\mathbf{B}/\omega\mu_0\epsilon_0)cos(\omega t)$ that is it's all consistent. So they must be out of phase...Grj23 (talk) 11:11, 30 April 2009 (UTC)

Apologies for the multiple edits. I guess the above only makes sense for a standing wave. For a travelling wave you might have for example

$\mathbf{E}(\mathbf{r},t)=E_0\sin(\omega t -kz)\mathbf{i}$ which gives $\partial\mathbf{B}/\partial t = \nabla\times\mathbf{E} = -kE_0\cos(\omega t-kz)\mathbf{j}$ and so $\mathbf{B}(\mathbf{r},t)=-k/mE_0 sin(\omega t-kz)\mathbf{j}$ so Heron was right in the case of a travelling wave, though not in the case of a standing wave.131.111.74.72 (talk) 12:21, 30 April 2009 (UTC)

It wouldn't be right to say the energy is stored "in places where the electric and magnetic field are not zero". The energy is stored in the changing EM field. The energy is in the change. An electron in an otherwise empty universe does not create energy nearby it from an electric field. It is in the change of the field which would have energy. For example, if the lone electron was jiggled by the hand of god (who isn't in the "universe" i.e. closed system), then an EM wave would be emitted.

Also, if the electric field of light is zero, then the magnetic field of the light will be zero as well - they are directly related. Fresheneesz 19:42, 12 April 2006 (UTC)

[edit] German version has a very nice pic

Would be great if there was a pic like that here too. The great German pic cannot be copied because it has many German words on the it. Andries 23:17, 3 January 2006 (UTC)

http://de.wikipedia.org/wiki/Elektromagnetische_Welle#Spektrum (here)
http://de.wikipedia.org/wiki/Elektromagnetisches_Spektrum (here)
http://es.wikipedia.org/wiki/Espectro_electromagnético#Espectro_visible (here)
http://es.wikipedia.org/wiki/Espectro_electromagnético
http://en.wikipedia.org/wiki/Electromagnetic_spectrum#Types_of_radiation
http://en.wikipedia.org/wiki/Electromagnetic_spectrum
http://en.wikipedia.org/wiki/Template:Electromagnetic_spectrum ... Ok! deWP, esWP & enWP Ok? --PLA y Grande Covián (talk) 13:58, 30 January 2009 (UTC)

[edit] Electromagnetic waves

How is it that electricity be considered electromagnetic wave? Electricity fails one of the fundamental properties of electromagnetic waves; that is, all electromagnetic waves travels through vacuum at the speed of light. So even though photons travelling through an optic fibre can be considered an electromagnetic wave since photons travel through vacuum at the speed of light. What is the justification for electricity travelling through a wire? How would you even apply Maxwell's equations to electrons? The first paragraph of this article seems to be in error.

Electricity itself is not necessarily an electromagnetic wave. However, electrical enegergy in a coaxial cable might be an electromagnetic wave, or might become one, especially if it is oscilating fast enough and if there is an antenna attached to the coaxial cable. See Radio frequency. --ssd 13:08, 12 January 2006 (UTC)

All electromagnetic waves do not travel through vacuum at the speed of light. They can travel through any medium except a superconductor, and except in a vacuum they travel slower than c.
All practical electrical energy transmission, whether in a vacuum or in wires, is by electromagnetic waves. If you relied solely on the movement of electrons in wires, then electrical energy would travel at about walking speed. In fact, wires just confine the electromagnetic wave to a narrow region of space, but it's still a wave. Even DC is a wave: when you close a DC circuit, the wave travels from the switch to the load until the whole circuit reaches equilibrium, and then the wave is absorbed by the load and disappears.
Maxwell's equations apply to electrons just as well as to anything else, but you'd need a very powerful computer to do the calculations.

--Heron 21:33, 21 March 2006 (UTC)

das ist verry güt der ist das klassa simmer im das schula und im die house das ist nict güt

[edit] Direction of propagation

From what (little) I understand of EM waves it seems as though they are always propagated in ALL directions; the front of the wave is a sphere in 3-space whose radius increases (at the speed of light in a vacuum) with time, hence EM RADIATion. I think it is for this reason that lasers are difficult to make as they confine the direction of propagation largely to a single spatial direction. This property of EM radiation would seem to completely destroy any conception of light as a particle (the photon) as the photon would "exist" everywhere in this growing sphere simultaneously. Rather it seems a photon should be thought of as the smallest burst of this spherical wave at a given frequency (according to quantum theory the energy of a single photon depends on frequency). -Kyp4

Lasers don't do what you say. They only produce a parallel beam when you put a collimating lens in front of them [1]. --Heron 18:30, 29 March 2006 (UTC)

That link is also partially incorrect, in fact. For a discrete beam, there's no way to make a truly parallel, absolutely collimated beam - diffraction will always spread the beam eventually. --Bob Mellish 18:45, 29 March 2006 (UTC)

This is something I find confusing too. Light is classically thought of as propogating outward in a sphere, as Kyp4 says, but a laser obviously doesn't do this. Is the article trying to say that the lens bends the partial sphere of light into a flat surface? If you have a point source emiting one photon at a time, what does the magnetic field around the source look like? Fresheneesz 19:35, 12 April 2006 (UTC)

The magnetic aspect of the em radiation we perceive is 90 degrees to the electro aspect. My question is: Is the magnetic aspect 90 degrees clockwise or anticlockwise to the electro aspect? -Gonegahgah (talk) 00:14, 27 October 2008 (UTC)

This is purely a matter of convention as to which way electric fields go an which way magnetic ones go. Martin Hogbin (talk) 20:07, 27 October 2008 (UTC)

Thanks Martin for your reply but what do you mean? Have they tested it? How do you test it? Were there results? -122.111.79.43 (talk) 22:11, 27 October 2008 (UTC) -Gonegahgah (talk) 22:16, 27 October 2008 (UTC)

[edit] Induction vs Hertzian Wave

There was a lot of confusion, even among top scientists, about whether various 19th century electrical experimenters working to develop wireless telegraphy or wireless telephony were producing anything beyond induction when they generated electrical transmissions of signals from an electrical arc or a simple coil of wire, and were able to produce sparks from a crude antenna nearby, or to receive audible voice frequency signals in a receiving coil connected to a telephone receiver. For instance, Edison used the term "Etheric Force" in the 1870's to describe his ability to detect a small arc between two carbon points with two metal squares as antenna, when a buzzer was arcing nearby. Leading scientists dismissed it as mere induction, ignoring Maxwell's equations, but the IEEE website now describes it as Hertzian wave transmission and detection. Hertz used a similar transmitter and detector, but did a rigorous demonstration of how the rf signals exhibited the properties predicted from Maxwell. Sources in radio history say that with induction the energy is localized, but Hertzian Waves travel a longer distance. A better "bright-line" distinction is needed. Various inventors like Preece and Stubblefield achieved wireless electrical transmission of voice signals in the late 19th century by induction. Does audio frequency current sent from a transmitting coil to a receiving coil technically constitute electromagnetic radiation, when at audio frequencies?Some help in the article for distinguishing what is and is not electromagnetic radiation would be helpful.Edison 22:14, 6 June 2006 (UTC)

Perhaps a test could be the following: for a coil of wire used as transmitter and another for receiver, each being a a certain diameter, with a certain number of turns of wire, at an audio frequency of say 1 khz, how rapidly does the induced voltage decrease as the colis are moved apart (their centers remaining coaxial). One falloff curve would be characteristic of electromagnetic waves, and a more rapid decrease would be characteristic of simple induction. Formulae I have seen for the mutual induction between two such coils indicate a very rapid falloff in mutual coupling when the antennas act as simply two windings of an air-core transformer.Edison 15:56, 13 June 2006 (UTC)

Yes, that might work. For more on the rates of attenuation with distance for the different types of field, see Near and far field.

There is another fundamental difference between induction and radiation. With induction, energy is transferred into the EM field during one half of the AC cycle and returned to the source during the other half. If a receiver is placed nearby, it picks up some of the energy which therefore does not return to the source. Thus, you can tell whether or not energy is being transferred by measuring the net amount of power flowing out of the source. With radiation, on the other hand, the energy flow is all one way: out of the source. None of it ever comes back, regardless of the presence or absence of a receiver.

A third way of looking at it is through Maxwell's equations. When you solve them for a radio wave, you get expressions for the E and H fields that are in phase, allowing them to radiate. In induction, they are out of phase, so they can't radiate. (For more on this, see Radiation from an Antenna by Peter Dodd.) --Heron 21:18, 13 June 2006 (UTC)

[edit] electromagnetic radiation and light

The first line of this article reads "Electromagnetic radiation (also more informally called light)" This must be corrected since light is only a specific frequency range. You could just as easily say "also more informally called radio waves" or "cosmic rays."

I agree. I think whoever wrote that was trying to make the point that some other parts of the spectrum near to visible light, like UV and IR, are sometimes called light, even though they're not visible - as in "ultraviolet light" and "infrared light". They then forgot that there are other types, like the ones you mentioned, that are never called light. The problem is that the boundaries between 'definitely light', 'sort of light-ish' and 'definitely not light' are subjective. Perhaps the criterion is that the radiation should behave like light under the conditions of whatever experiment you are doing at the time. Unfortunately, the American Heritage^® dic·tion·ar·ies says, in definition no. 2, "EM radiation of any frequency". [2] --Heron 18:36, 18 June 2006 (UTC)

P.S. The OED, in subsense (f) of sense (d) of light, n., includes a quote from Maxwell himself: "...light itself including radiant heat, (and other radiations if any), is..." Perhaps Maxwell was the first and last person to use the word in this way, and all subsequent dictionaries have blindly copied him. He didn't know much about the EM spectrum at that time - not even about radio. --Heron 18:43, 18 June 2006 (UTC)

In my experience, most physicists have no problem using light for EM radiation outside the visible (especially with regards to non-visible laser light). — Laura Scudder ☎ 19:04, 18 June 2006 (UTC)

Even though light can be defined with electromagnetic waves of any wavelength, I still think starting the article with that statement just comes off as uneducated. The most important thing is that people who are not physicists who read this article understand light to be visible light and nothing else. Keep in mind that those of us who understand the different wavelengths of light already know that EM radiation is light so it will not matter if that is taken out, but for those who don't know, that statement is just confusing. -(guy who started this)

I tried to avoid the whole "what wavelengths are exactly light?" discussion by just changing it to "sometimes informally called light". I don't know if that addresses the above complaints, but I just thought that delving into the semantic debate in the lead was something to be avoided. — Laura Scudder ☎ 03:52, 20 June 2006 (UTC)

I just avoided the problem even more by deleting the statement. I left in the word light, but only as part of a list. I think the best place to discuss the "what is light?" question is under Light. People who want to know about "informal" usage are more likely to go to that article first. --Heron 20:22, 22 July 2006 (UTC)

I have a question why can't we see other forms of radiation? What is necessary for our eye's to be able to see all forms of electromagnetic radiation/light? I think it's because only a range of radio waves and visible light can penetrate the atmosphere, the others don't, fortunately. It begs the question why beings with eyes cannot 'see' radio waves as well. I don't know the answer.--Robert van der Hoff 00:51, 8 May 2007 (UTC)

Mainly because we have set detector cells in our retinas (rods and cones on the outer retina, and photosensitive melanopsin ganglion cells on the inner retina) that are designed to "detect" visible light. Some animals can see in infra-red I believe as well. Birds and bees can detect the earth's static magnetic field, and use both it and visible light for navigation. Clearly bits of us are just designed for set purposes ;) Topazg 16:57, 29 June 2007 (UTC)

[edit] The Null Effect of magnetic or electric fields on EM radiation/light

The opening sentence: "Electromagnetic radiation, sometimes informally called light, is generally described as a self-propagating wave in space with electric and magnetic components" is misleading because light or EM rad is never deflected or affected in any way by a weak or strong static electric field or a permanent magnetic field located in a vacuum or space.

Shall we change this sentence. bvcrist Bvcrist 20:52, 10 July 2006 (UTC)

We can't change the sentence, because it's correct. We could, however, explain that light is not influenced by static fields in a vacuum, as you say. --Heron 19:45, 11 July 2006 (UTC)

Mass and frequency are equivalent, a photon carries a change of frequency between the individual charged particles.

--79.67.155.20 (talk) 11:02, 5 May 2008 (UTC)

[edit] All of the EM spectrum is light?

When did physicists start to make this assertion? At the start of the article we have "In some technical contexts the entire range is referred to as just 'light'." The given reference is not a good one. It points to a website of an organisation that most pepople have never heard of, and if the assertion is made, it's not easy to find. Any physicists out there? Please come up with a much better reference than this. Maybe we should put it as needing a citation. In any case, physicists can't just change the definition of light. That's down to people who write dictionaries. I think most dictionaries give a definition of light as being something that is visible - to a natural eye. Are physicists wrong (what a question - are they ever) ? Arcturus 21:16, 20 October 2006 (UTC)

To define light as only the part of the spectrum visible to human eyes might be common in general use and in dictionaries, but its at least very common in Physics to also include nearby parts of the spectrum (ultraviolet, infrared) that are affected by the same phenomena, or that produce the same effects, or can be analyzed with the same mathematical tools. For example, "infrared light" can be focussed by lenses, detected by semiconductor photodetectors, and analyzed by ray tracing very much like visible light.

I don't think its very common for anyone to refer to AM radio frequencies, for example, as light, though there may be certain contexts where its useful to do that. For that reason, an optics text might very well mention that the whole spectrum can be considered as "light" at least in some situations.

To me, the term "light" is the part of the spectrum that was historically analyzed by ray optics or geometric optics. This developed long before the rest of the spectrum was even discovered: Modern telescopes were invented in the early 1600's, but the rest of the spectrum wasn't known until Heinrich Hertz's discovery in 1888.

Finally, any modern description of "light" or "optics" must also include the full electromagnetic analysis to describe things like diffraction. This makes the boundary between light and microwave and rf radiation very blurry. But the need to see light as part of the electromagnetic spectrum developed historically long after the word "light" was defined.

I agree the article should not claim that all of the spectrum is "light" (otherwise, we should merge Light and Electromagnetic radiation). But it is definitely correct to include some of the spectrum beyond the visible spectrum.

-- The Photon 22:09, 21 October 2006 (UTC)

Agreed, UV and IR are commonly referred to as light. Should we then clarify this in the article and adjust the text so that it doesn't imply that the whole EM spectrum is regarded as light? Arcturus 12:34, 26 October 2006 (UTC)

I think the usage of the word light is appropiate. I think the name of the area on the EM spectrum referred to as 'visible light' should change. Just as the other areas of the EM spectrum have technical names, that area should be given a better technical name, as well. My reason being, all wavelengths on the spectrum are comprised of photons. When we hear the word photon, the words photo and camera come to mind. A camera captures light and photo film is light sensitive. I'll leave the name change up to scientists.Eevathediva (talk) 07:21, 23 April 2008 (UTC)

[edit] Explanation for a Grunt Lost in Space

Just what exactly is an EM level? How is it measured in regard to human physiology? If one turns 90 degrees and faces into the the four different directions: North, South, East, or West, or even up or down would a person's EM level vary? If so why? Please reply. Be gentle and kind. It may not look that way, but I am very fragile. User:Kazuba 31 Jan 2007

[edit] Misleading mention of AM and FM in diagram

The way that "AM" and "FM" appear in the wavelength diagram and the fact that it is otherwise devoid of specific mentions of applications for various bands of wavelength (e.g. microwave oven, television) could easily fool an uninformed reader into thinking that AM and FM are synonymous with specific parts of the radio wave spectrum rather than correlating with them. --AceMyth 03:27, 16 February 2007 (UTC)

[edit] Merge

This merge request should be removed. Light = Radiation, but Radiation != Light.

Arrgh. Forgot to sign. Real programmers know that 1 and 1 makes 10. E 9 07:23, 3 April 2007 (UTC)

My concerns looking at electromagnetic radiation and light are that it is not clear that light is talking about visual light, while electromagnetic radiation is talking about electromagnetic radiation. Feel free to take the tag if the definitions are defined distinctly.100110100 07:30, 3 April 2007 (UTC)

[edit] Error on diagram

The label $102$ appears twice on the wavelength axis of the diagram. Peter Harriman 06:29, 21 May 2007 (UTC)

You are right; there's a minus missing. On my Firefox (Mac 1.5.0.11) the diagram completely fails to show up at all, and I just see a white box. Dicklyon 15:05, 21 May 2007 (UTC)

I attempted to fix it, but my SVG-fu is weak, doing so seemed to break the image completely. (Actually, the ways of the server-side SVG renderer are mysterious, and seem to result in a white box at least half the time, even when files will show correctly when rendered locally.) --Bob Mellish 15:42, 21 May 2007 (UTC)

Thanks to Sakurambo, the creator, who has fixed it! Peter Harriman 20:07, 21 May 2007 (UTC)

[edit] Electrosmog

Hi, I'm trying to move the info from the electrosmog article to this, so that we can get rid of that rather poor effort. Unfortunately, other than the definition of the term there is nothing much useful left. The supposed effects on human health are covered elsewhere, and the natural world effects are a media non-story. I put the definition in the intro, but maybe that's too high. I don't think it warrants it's own subsection, but I'll leave that up to you guys and gals. 128.243.220.41 15:05, 28 June 2007 (UTC)

Having spent quite a bit of time trying to get some general agreement to have electrosmog as a redirect only and get any relevant information concisely written on the electromagnetic radiation page, I would like to ask first that it stays here instead of having to have another article for it, and secondly that it doesn't have its own heading, as it is not deserving of one. I actually think having it at the top (perhaps with suitable intro: "Recent coverage in the media has referred to electromagnetic radiation as "Electrosmog".....") and then the text that we currently have would be the best way of having it in the article. Topazg 16:51, 29 June 2007 (UTC)

I think what's written is supremely neutral, but as the term IS used is a negative sense and pretty much always in relation to possible health concerns, shouldn't there be a link to said concerns? 129.215.141.101 13:56, 1 August 2007 (UTC)

To whoever keeps putting it in there, please note that the word electrosmog is not a portmanteau word. Electromagnetic is shortened to the standard prefix electro-. Smog is not shortened at all. There is no blending of words, and electrosmog is no more a portmanteau than electromagnetic. Sbacle 18:09, 6 September 2007 (UTC)

[edit] Accelerated electrons

Could any of you Experts out there put up something about the quantum explanation of why an accelerated electron radiates?

I know the classical maths indicates that it happens, though not everyone seems to agree - Feynman among them I believe; he says it is rate of change of acceleration apparently - , but no one seems to have a quantum explanation.

I thought I was on to it when Stumble introduced me to Hydrino, but contributors there seem to agree in nothing but disagreeing with Mills.

So come on someone, and sort out all those struggling engineers who can't see how an antenna works! And while you are at it, how does a photon know if it is part of the induction field or the radiated field?

--Boletusedulis 22:35, 24 July 2007 (UTC)

[edit] Direction of propogation

Just wondering here, if the direction of propogation is ExB as mentioned at the end of the article, should the direction of propogation for the wave in the first image be from right to left and not left to right, as is currently the case? EZG 14:10, 1 August 2007 (UTC)

Yes, you are correct. — Laura Scudder ☎ 15:19, 1 August 2007 (UTC)

[edit] Effect of Rare Earth Magnet on EM Radiation: None !!!

If EM radiation is the propagation (or self-propagation) of EM fields, then why does a strong magnetic field have no effect on the direction of EM radiation? Surely the EM fields of light are affected by strong magnetic or strong electric fields, yet no one has published such effects. Strange isn't it.

In my world, light is generated by EM fields, but it is does not have electric or magnetic properties as it moves through space. When light hits a material, the EM fields of the material are able to interact with light, but that does not prove that light has and possesses EM fields as an inherent property. Light, at all wavelengths, does not have any EM properties. It is purely energy, which is a form of matter or something else, that no-one yet recognizes or understands.

Vince Crist Jan 7, 2008 —Preceding unsigned comment added by 64.165.113.102 (talk) 08:39, 7 January 2008 (UTC)

Strong electromagnetic fields do affect light, but they have to be stronger than the field of your magnets, and the mechanism involves virtual charged particles. Try searching for "photon-photon scattering" online. Melchoir (talk) 09:12, 7 January 2008 (UTC)

Send me the paper and I'll shut up. Otherwise you are simply a Catholic Pope trying to shut up Galielo. Vince —Preceding unsigned comment added by 64.165.113.102 (talk) 09:23, 7 January 2008 (UTC)

Sweet, I get to become a world leader, and all I have to do is ignore you? Thanks! Melchoir (talk) 09:25, 7 January 2008 (UTC)

Well HotDog, where's my paper, where's the link? Don't be shy or lazy. Prove to me that you have some proof to back up your claim. Com'on send the paper or send the web-site link. Don't be a PrimaDonna like all the wanna-be Physics Grad students and their inability to learn by or think for themselves. As you surely noticed, I don't hide behind a monniker as do nearly all Wiki-editors and would-be maintainers of modern day misconceptions. My name is Vince Crist and my e-mail is: bvcrist@xpsdata.com. Want a phone number? —Preceding unsigned comment added by 64.165.113.102 (talk) 19:44, 7 January 2008 (UTC)

Vince - although I appreciate your enthusiasm, this is not the place for this kind of discussion. This is intended for discussion of the article. If you would like to discuss theories and ask questions, there are more appropriate places. Good luck with your quest. PhySusie (talk) 20:53, 7 January 2008 (UTC)

Susie - I am discussing the article by addressing the adjectival description that has no basis in fact or proof of any sort that is published anywhere. Since there is no proof as per Wiki laws for the verifiablility of that adjective, why is it written here. Everyone just accepts it with no proof. Show me the original proof or some recent proof is all I ask.

Maxwell died before Hertz produced his result. Hertz never proved that his microwave radiation had either magnetic or electric fields as it propagated through the room to his receiver. Hertz did use as large metal mesh to collect some of the radiation, but he did not bias the mesh in any way except to rotate it by 90 deg. Who in history decided to define light as being and having electric and magnetic fields? I'd love to know. If you have an article, book or recent test that shows that light has either or both electric and magnetic field properties as it moves through the air, I'd love to read it.

I've been meaning to do another little, potentially very interesting, experiment. Run a small DC circuit with a micro or pico-ampmeter and place a rare earth magnet next to one of the wires to see if there is any effect on the current. If there is a drop, does it level match expectation? If you keep the magnet in place, and have a thin wire with a high current, you might be able to melt the plastic on the wire or heat up a bare metal wire. Care to try? Vince Crist Bvcrist (talk) 08:01, 16 January 2008 (UTC)

"Surely the EM fields of light are affected by strong magnetic or strong electric fields" Visible light is caused by photons. Photons are also responsible for magnetism. A rare earth magnet just isn't strong enough. —Preceding unsigned comment added by 69.18.178.18 (talk) 08:30, 23 April 2008 (UTC)

OK, I found this little discussion quite intriguing and it got me thinking... I'm not convinced that a non-varying magnetic field would have any effect on the propagation of a photon due to the superposition principle. But I read here that people are doubting the integrity of Maxwell's equations; unless I have mis-interpreted it, it asks here who decided light consisted of electric and magnetic fields and that photons have no EM properties themselves. Well this was shocking to read, if one were to read most University-level texts on waves and the derivation of em wave is a frequent occurrence (one such excellent text, in my opinion, is Optics 4th E.d (HECHT, E.) - that I mention later in the thread). EM waves are the solution to the wave equation consequential to Maxwell's equations. It describes how disturbances in em fields propagate in space-time. The interaction of light and matter depends on the fact that em-radiation has both electric and magnetic components and that dispersion occurs from the ability for the induced dipoles in a dielectric substance to keep up with this oscillating field. --Ukberry (talk) 06:52, 1 May 2008 (UTC)

[edit] Ambient electromagnetic radiation

This is a tough request, but it would really help this article if someone could find a chart of the ambient electromagnetic radiation over the entire spectrum in some ordinary location (the middle of Central Park on a sunny day, for example). I mention this because the statement in the text

"Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths."

doesn't sound right to me. My guess is that sunlight would be a huge peak even on a log graph, which would help explain why that sliver of spectrum is so important, perhaps rivalled by some of the IR, and that even in a big city the total radio energy would be pretty tiny in comparison, while many of the more exotic frequencies would be practically empty. But I could be wrong - I can't recall ever having seen such a helpful comparison. 70.15.116.59 (talk) 00:58, 15 January 2008 (UTC)

The statement is correct. Sunlight contains wavelengths across the electromagnetic spectrum, not just visible light. Although our atmosphere shields us from much of it, more than just the visible part makes it to the surface of our planet. I am not aware of any measurement of the different parts of the spectrum at a particular location.PhySusie (talk) 17:42, 15 January 2008 (UTC)

Well, Planck's law has a relatively sharp peak, and the sun really is yellow, not just within the visual spectrum but in the absolute sense. I think the yellow peak should stand out plainly over all the rest, while as for the radio transmissions of the sun... you can see a 100-watt radio transmitter a lot further away than you can see a 100-watt light bulb. (The atmospheric window also imposes some of those limitations) 70.15.116.59 (talk) 03:24, 16 January 2008 (UTC)

Yes - em radiation from the sun peaks in the yellow part of the visible spectrum - though what you mean by "not just with the visual spectrum but in the absolute sense" is not clear. And I think I did state that the atmosphere blocks certain parts of the spectrum. I'm not sure what you mean by seeing a 100W radio transmitter better than a 100W light bulb - but that is besides the point. The original statement is still correct. The sun is not the only natural source of em radiation - and natural sources do emit across the spectrum regardless of their intensity. And our technology can manipulate across the spectrum as well. Both sentences are correct. I don't see any reason to change it. PhySusie (talk) 15:52, 16 January 2008 (UTC)

[edit] A clarification

Are there any particles emitted, when radiation is created? I was always under the impression that light = stream of photons, but the article does not mention it anywhere. Thus I wonder what does actually radiation consist of? If it consists of "nothing" and is just a wave (=the light is not transferred by any particle) wouldnt it mean that vacuum in fact consists of some "invisble" bricks that interact with each other? (e.g. when you put 5 billard balls in a line and hit the first one, the 4 other balls will move too). Agameofchess (talk) 19:49, 10 April 2008 (UTC)

Light is transfered by particles. Photons. All radiation is particles. All the various forces also have corresponding particles, except maybe gravity. —Preceding unsigned comment added by 69.18.178.18 (talk) 08:32, 23 April 2008 (UTC)

Actually the question is too deep for such a flip answer. Light can not be completely described as either particles or as waves. And no, there are no interacting "bricks" in space; that's the old luminiferous aether theory that was displaced by Einstein. Read about photon and wave–particle duality. Dicklyon (talk) 00:13, 12 May 2008 (UTC)

[edit] Serious Diagram Error

The wave diagram just after "Properties" has a serious error. Magnetic and electric fields are 90° out of phase and in quadrature. Both do not go to zero at the same time.Trojancowboy (talk) 21:07, 14 April 2008 (UTC) Trojancowboy i9s not alone. Not an error. The first of 20,000 Google hits for "Transverse Electromagnetic Wave", www.play-hookey.com/optics/transverse_electromagnetic_wave.html makes the same mistake, and sticks to it although I tried to correct him years ago. This error is pervasive, and is part of the unrecognised crisis, part of which I discuss at www.electromagnetism.demon.co.uk/17136.htm . - Ivor Catt (talk) 21:11, 26 July 2008 (UTC) Ivor Catt

[edit] Several Errors

Agree with above comment - The magnetic and electrical components of an electromagentic wave are out of phase with each other by 90deg. To answer the question regarding energy and oscillation it is measured by the RMS value.

Please alter reference to 'Light' when refering to that range of frequencies that the human eye can detect. Light = Electromagnetic Radiation. Those frequencies which the human eye can see are more correctly termed 'Visible Light'.

Alternating electrical currents are not a flow of electrons. Please refer to electrostatics for more information.

Electical currents (individualy) do not produce electromagnetic radiation and neither do magnetic fields: Electrical currents induce a magnetic field and vice versa. Your explanation of electrical current traveling down a wire producing electromagnetic radiation is very missleading. It is the resitive effect of the wire that produces the EM radiation. —Preceding unsigned comment added by 62.6.149.17 (talk) 14:30, 16 April 2008 (UTC)

[edit] COMMENT - Disagree with the comment and response to comment

The electric and magnetic fields are in fact in phase in EM light. They are out of phase in AC circuitry. Eugene Hecht has the best book on Optics I've read through my academic career in Physics and when referring to electromagnetic radiation zone he states with mathematical analysis:

"In this zone, a fixed wavelength has been established; E and B are transverse, mutually perpendicular, and in phase" [1]

A more useful thought experiment I thought about when asking myself what would happen if they were out of phase setting up normal modes. If they were out of phase, the nodes of the electric field would be at different locations to the magnetic ones; so you would be able to couple energy from these points, so interference patterns would not occur. So if the components are out of phase in EM radiation, they would behave like EM waves in a circuit (which do not exhibit interference in the same way).

The reason I disagree with the diagram is that, although the direction of propagation is not explicitly noted (it is assumed to travel with increasing distance) and base vectors are not defined (assume E field on the +y axis and M field on the +x axis with propagation down the +z axis), the light would not be travelling in this direction on the diagram.

The reason I disgree with:

Electical currents (individualy) do not produce electromagnetic radiation and neither do magnetic fields

is that this is the basis of telecommunications and optoelectronic effects. Radio and TV use this effect all the time.

[1] - HECHT, E. Optics 4th Ed. (2002)

--Ukberry (talk) 14:25, 27 April 2008 (UTC)

Ukberry is correct (I've just checked in Hecht myself). The waves are in phase. Now why in the world did I have in mind that they were 90 degrees out of phase?Headbomb (talk · contribs) 17:49, 27 April 2008 (UTC)

All, see the Maxwell's equation. Faraday's law and Ampere's circuital law are responsible for EM waves propagation. They state (approximately) that curl of Electric field is equal to the rate of change of Magnetic field and vice-versa. curl is the rotation vector (which is itself defined in terms of differentials). Hence E and B will be in phase!! Ahirwav (talk) 13:08, 2 May 2008 (UTC)

Ahirwav, are you implying that if ANY two fields are related by the time derivative of the other, then then their solution to a wave equation (where space and time are considered) must be in phase??

--Ukberry (talk) 13:54, 2 May 2008 (UTC)

A possible resolution of this problem is to say that the electric and magnetic components are 180 degrees out of phase, noting that the polarity of the fields (N/S, +/-) is purely conventional.86.132.189.39 (talk) 20:47, 27 July 2008 (UTC)

[edit] Serious phase errors!

That there are widespread and serious errors with regard to Maxwell's equations and the creation of EM waves can be seen by the above discussions. It is also seen in the widespread error that Electric and Magnetic fields create each other. THEY DO NOT! Also the Electric and Magnetic fields ARE IN PHASE as was indicated above. I corrected this error in the article which was quickly returned to its erroneous state (as expected). I provided proof of my assertion with reference to the actual mathematics of the causality of Maxwell's equations and the conclusion that the equations that everyone believes show that E and M fields create each other are NOT in fact causal. This calculation and the arguments it is based upon, was done by O.D. Jefimenko for which I provided the reference. This same conclusion was also arrived at by Panofsky for whom I did not provide a reference. Indeed the 90 degree phase shift concept arises because of the mistaken belief that E and M fields "create each other". Clearly there is some determination here to insure that these widespread errors remain included in this article. The mathematics and arguments of Jefimenko and Panofsky do not appear open to debate and absolutely none of the widely accepted physics and E&M textbooks show a 90 degree field shift. (And neither do the illustrations in this article) I don't think that proven erroneous concepts such as the E and M creation story or the 90 degree phase shift error have any place in a quality article. I don't think that a trick of saying that these fields are "out of phase" (which is obviously NOT 90 degrees)by cleverly assigning conventions represents an honest approach to this matter either. —Preceding unsigned comment added by 69.218.219.89 (talk) 20:31, 17 June 2009 (UTC)

Why are you talking about a 90 degree phase shift between E and B? Of course, everyone knows that E and B are in phase in a linearly-polarized EM plane-wave. There's no controversy about this. Is this error in the article somewhere? I don't see it... --Steve (talk) 21:43, 17 June 2009 (UTC)

Mentioned it for two reasons. One is that in these comments above someone claimed that there was a 90 degree lag. This is a common misconception due to the lag in found in resonant electrical circuits. But there is also another important point. Electric and magnetic field clearly are occurring simultaneously! The principle of causality clearly states that all events MUST be preceded by a cause. That means the cause must be prior in time. Simultaneous event CANNOT "cause each other! The article plainly states that the electric and magnetic fields are causing each other. This is obviously a totally incorrect statement although it is also a widespread error due to the false assumption that Maxwell's equations as usually expressed represent causal relations. They do not (for the most part). This error has been clearly shown by Jefimenko as well as Panofsky whose publications provide ready reference for this assertion. OK? —Preceding unsigned comment added by 69.218.235.59 (talk) 19:17, 18 June 2009 (UTC)

Oh, OK. I wouldn't worry about the above comments that there's a 90 degree lag. You can find all sorts of nonsense on a wikipedia talk page, it doesn't mean it's a "common misconception".

When I kick a ball, the ball accelerates. One might say that the force of my foot causes the acceleration of the ball. But the force and the acceleration are at the same time, F=m*a. Right?

Say I have a capacitor, which I repeatedly charge and discharge with some frequency. Most textbooks would say that the charges on the capacitor plates create the electric field within the capacitor, and that changing electric field causes a magnetic field within the capacitor. Jefimenko, I guess, would say that the charges on the capacitor plates create both the electric and the magnetic fields within the capacitor. I don't think one of these points of view is correct and one is incorrect. I think they're different ways to think about the same event.

If you intend to create a changing electric field, you will also create a magnetic field, whether you intended to or not. I think that's what people mean by "a changing electric field causes a magnetic field". Obviously no one is saying that the electric field has a force of will and intentionally creates the magnetic field, or that the electric field needs to have been changing for 10 milliseconds before the magnetic field finally gets a chance to start responding. And it can also be the case that the magnetic field can cause a changing electric field.

Maybe a better example is a resistor: Sometimes it makes sense to say that the voltage across a resistor causes a current to flow, sometimes it makes sense to say that the current across a resistor causes a voltage drop, and in reality the current and voltage are simultaneous. These aren't contradictory. Do you agree? --Steve (talk) 22:33, 18 June 2009 (UTC)

No, both views of a capacitor are NOT correct. If one accepts the law of causality, then clearly the charges create the electric and magnetic fields simultaneously which then travel at the speed of light from those charges to where they are going. Since they are simultaneous it's impossible for them to "cause" each other without violating causality!

The whole problem here (and it IS a common error) is to look at Maxwell's equations in the usual form which obviously seem to show that a changing electric field dD/dt seems to be "causing" H and and a changing magnetic field dB/dt seems to be causing E. It seems so obvious! But the problem is these equations represent equalities NOT a causal relationship! The quantities can be EQUAL but they do not "cause" each other! Indeed, both electric and magnetic fields are caused by charges and their changes. They are created simultaneously and thence proceed out into space at the speed of light. Maxwell himself seemed aware of this problem using the term "measured by" rather than "caused by" in certain non-causal situations. (see Maxwell section 531)

Of course STATIC charges can create an electric field without a magnetic field and a constant current can create a magnetic field without an electric field, but these are not the cases involved with EM radiation where both electric and magnetic fields are changing with time.

Now look at the section in question:

"According to Maxwell's equations, a time-varying electric field generates a magnetic field and vice versa. Therefore, as an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. These oscillating fields together form an electromagnetic wave"

It CLEARLY implies that we are going to use the above erroneous conclusion and implies that the way electromagnetic waves keep going through space is sort of like the way energy exchanges back and forth between a capacitor and Coil in an LC circuit. Yes, they don't actually SAY that but the implication is there. And of course the difference is that in the LC electric and magnetic energy DO have a phase shift between them whereas in EM waves both are IN phase. That makes all the difference in the world! One might argue about the semantics of the word "generate" vs "create" but to me they both seem to imply the same thing. Namely that electric and magnetic fields are "creating" each other, which they do not do.

Hence a correct statement would be that a "source" of time-varying charges and/or currents (a flow of charge) simultaneously creates electric and magnetic fields which travel through space together at the speed of light to form an electromagnetic wave. The statement as it now stands is simply incorrect.

To consider the resistor example, to put a voltage (potential) on the resistor means to bring charges from infinity to the ends of the resistor. Once those charges stop moving they create an electric field in the resistor. Note that the Electric Field proceeds from the charges down the resistor at the speed of light! That electric field then creates a force upon the static charges within the resistor causing them to accelerate. As this occurs current begins to flow. Hence to say that a voltage "causes" a current in a resistor is correct! They do NOT happen simultaneously. To say that the current "causes" a voltage drop in the resistor would not be true and we see the voltage happens before the current does. However, in a practical case we do know that there is a law (Ohm's law) which relates voltage and current in a resistor. But as above Ohm's law ONLY represents a causal relationship if it is written in the correct form. We would also observe in this case that once the current is flowing in the resistor, it creates a magnetic field which proceeds away from the resistor at the speed of light. So in this case we see that electric and magnetic fields are NOT created together and in fact one could say that the electric field DOES indeed create the magnetic field (but not vice versa). But in this case to be correct we would like Maxwell have to say that the voltage drop in a resistor is "measured by" the current in it, rather than "created" by it!

The problem here is that because things happen really fast there is the tendency to assume they are actually at the same time. In a practical case that would be essentially true! But when one starts insisting that this or that CAUSES something else, one has to be very careful to figure out what comes first, because simultaneous things cannot "cause each other." —Preceding unsigned comment added by 69.218.211.181 (talk) 17:28, 6 July 2009 (UTC)

So you would agree that there's cause and effect in an LC circuit? What's causing what? As I'm sure you know, the voltage across the inductor and the voltage across the capacitor are in phase... --Steve (talk) 18:21, 6 July 2009 (UTC)

[edit] Old Revision Reverted (Opening Paragraph)

I was confused with the edit at first. Need to talk about this. So I don't get the word phenomenon was removed, this clarifies that light is "an occurrence of fact" (from Dictionary Definition), where the fact lies in the physics of the system.

Then the verb "perceive" was removed and replaced with sense; however the rewording could imply that the eye could perceive other phenomena. The eye certainly does perceive, defined as: "to become aware of (something) through the senses". Thus, we get a more concise description.

I must also clarify that a disturbance does not require a physical "medium", the disturbance with EM radiation is a disturbance of the EM field that is present everywhere in space; thus, I have reverted the edit.

I also want to see if everyone is happy with the diagram with the text and remove the dispute of factual accuracy; although I think that a new diagram would be better (would do it myself, but I'm not so good at graphics). And the sections on wave and particle models, I want to have a link between the two - why we observe both models.

Can I have some feedback please

Dictionary Definitions from: Collins Online Dictionary

--Ukberry (talk) 00:17, 12 May 2008 (UTC)

[edit] "Eighty-one octaves"

So does that mean that there is a minimum or maximum energy level (frequency) for EM? I was under the impression that there were theoretically no limits.

Stonemason89 (talk) 23:39, 13 July 2008 (UTC)

There may be some theoretical limitations to the frequency of EM radiation but the reference cited does not seem to give any and I suspect that the maximum possible range is more than 81 octaves. I suggest that the statement is removed until it can be better justified.Martin Hogbin (talk) 17:04, 14 July 2008 (UTC)

A little research has shown that cosmic gamma rays up to 100 GeV have been detected. I see no problem with EM radiation with uHz frequencies which gives us a range of a little over 81 octaves. A fundamental limit on the range might be set by the number of Planck lengths in the observable universe, which is estimated at 2.7 × 10⁶¹ - over 200 octaves. I will therefore remove the statement until someone has value which can be verified.Martin Hogbin (talk) 09:43, 16 July 2008 (UTC)

I have just noticed that the 81 octave statement cites Isaac Asimov's Book of Facts. I suspect that this is the range that been observed rather than the maximum theoretically possible. I will still remove the statement pending clarification.Martin Hogbin (talk) 09:48, 16 July 2008 (UTC)

[edit] Properties Diagram Is For t = constant?

Is the Properties diagram meant to portray a snapshot of a wave at a fixed time or is it meant to imply that you could stand at a point on the distance axis where the E and M components are zero and never detect the wave no matter how long you waited? If it is a representation of a function like sin(A*distance + B*time) then I think something should be added to the description of the diagram to say that time is constant.

Tashiro (talk) 17:39, 19 January 2009 (UTC)

[edit] Informal RfC for 'Speed of light' article

There is a dispute concerning the wording of the 'Light as electromagnetic radiation' section of the 'Speed of light' article. Editors are requested to give their opinions on the 'Speed of light' talk page. We decided to ask on related article talk pages rather than go for a full RfC so that we would get editors with a knowledge of and interest in the subject. Martin Hogbin (talk) 18:28, 6 February 2009 (UTC)

[edit] Why is this article called Electromagnetic radiation, not Electromagnetic wave?

I am curious about this. Matthew 百家姓之四 Discussion 討論 04:33, 23 March 2009 (UTC)

[edit] Merge with light

Making a distinction between EM radiation and light seems a bit POVish to me. The only purpose seems to be to make a distinction between light and visible light. But there already is an article on visible light. Yes there are a number of cultural elements in the light article that don't really belong in a scientific article about light but these could be merged with visible light. Serendi ^pod ous 08:13, 18 May 2009 (UTC)

I will say that, in my opinion, rather than Electromagnetic Radiation, Light, Visible Spectrum, and Electromagnetic Spectrum, we should have Electromagnetic Radiation, Visible Light, and Electromagnetic Spectrum. (That is, merge "Light" with "Visible spectrum".) As a physicist, I feel that, broadly speaking, using "light" for "electromagnetic radiation" is sometimes appropriate and sometimes not. If the situation pertains to "almost-visible" (near-IR and near-UV) radiation which interacts with matter much like visible light and can be manipulated with similar optics, it is usually acceptable to call it "light". Also, when discussing phenomena that are highly applicable to visible light (e.g. Rayleigh scattering or refraction), it is justifiable. This may include those applicable to any electromagnetic radiation, such as the "Doppler shift of light", "polarization of light", etc. However, in the context of physical phenomena occurring only far from the visible portion of the electromagnetic spectrum, it sounds highly odd to use the word "light" for the electromagnetic radiation involved. An example would be Compton scattering of X-rays and gamma rays: we simply do not say "Compton scattering of light", as the phenomenon is not observed for visible (or almost-visible) light. (But yet physicists do get away with (ab-?)using the prefix photo- as in photonuclear reaction or pion photoproduction even though they are talking about a reaction induced by gamma rays!) So I would say it's more of a style issue when to use "light" for "electromagnetic radiation". 69.140.12.180 (talk) 21:19, 26 May 2009 (UTC)Nightvid

Another peculiar case is the Advanced Light Source and Advanced Photon Source, both of which produce mostly X-Rays. :-) Anyway, I agree with 69.140.12.180: Merge visible spectrum and light into a new article: visible light, about the past and present understanding of visible light. That's the common-sense, near-universal definition of the term "light". Even 800nm photons are normally called "infrared radiation", not "infrared light". Of course, much of the current light article would move here instead. --Steve (talk) 07:31, 27 May 2009 (UTC)

Agree. Merge light into visible light, with "light" redirecting there. Jheald (talk) 10:54, 27 May 2009 (UTC)

Disagree. The Visible spectrum article is about the spectrum, not about light; it can logically stay separate from the light article, which covers many aspects of light unrelated to spectrum. Dicklyon (talk) 15:43, 27 May 2009 (UTC)

On second thought, I agree with Dicklyon: "visible spectrum" is worthy of a separate, narrowly-focused article, about the different wavelengths from 400nm to 700nm, and their relation to color and other physical phenomena. How about just making visible light redirect to light instead of redirecting to visible spectrum? (Plus, moving some content from visible spectrum to light.) --Steve (talk) 19:41, 27 May 2009 (UTC)

Oppose - The current division seems broadly correct: Light (a less technical article, using the common or garden term and focussed on the part of the spectrum that we can see) and Electromagnetic radiation (more heavy-duty physics content, discusses things which are common across the whole spectrum). Perhaps these articles could be augmented with content from Visible spectrum and Electromagnetic spectrum, and I agree with Steve's suggestion that Visible light should redirect to Light, but that's separate to this merger proposal! Djr32 (talk) 22:07, 27 May 2009 (UTC)

I would support merging Visible spectrum with light, and merging electromagnetic spectrum with electromagnetic radiation. Both article pairs seem to have a large overlap. I would oppose merging light/vis. spectrum with EM spectrum/EM radiation. I think visible light, as the stuff we see, has an important distinction from the rest of the spectrum. It also gives a place for a slightly more physics-light description - I do not think this page needs to go into QED. The EM radiation page can then go into more depth, and of course cover the full range. The light page would link there for more details. GyroMagician (talk) 10:27, 3 June 2009 (UTC)

Given the lack of enthusiasm shown for the original merge proposal, I have removed the merge tags. There were a number of other suggestions above for articles which could be merged, which could be followed up on. Djr32 (talk) 17:41, 6 June 2009 (UTC)

There seems to be consensus that a merge is necessary, but no consensus on what that merge should be. I would prefer it if such a merge were conducted by someone with a working knowledge of the subject. Serendi ^pod ous 19:10, 6 June 2009 (UTC)

Oppose - Electromagnetic radiation includes radio waves and microwave energy, which are NOT considered light. Light is a form of electromagnetic radiation but NOT all electromagnetic radiation is light. --96.225.33.226 (talk) 21:53, 28 June 2009 (UTC)

[edit] EM wave quantized

The article states that "energy of an EM wave is quantized." This clearly is not true, although it is a common misperception. What is true is that energy is added to and removed from the EM field in quanta because this (the conversion process) involves matter which gives up and accepts energy in discrete "quantized" amounts. No citation is given for the EM wave itself being quantized. Maxwell's equations for EM waves do not (and can not) predict any quantization. blackcloak (talk) 19:19, 28 July 2009 (UTC)

This is a common correct perception. :-) I added a ref.

I read the section starting on pg 511 of the reference. To me the logic appears circular. The cited experiment does not confirm the quantization of the EM field; it merely confirms conservation of energy in the conversion from the EM field to the various energy components of ejected electrons. Stated another way, an analysis of the remaining energy of an electron emitted from a surface of a material with a work function in no way says anything about the quantization of (i.e. particle nature of) the EM field itself; it does say something about one (frequency) component of what might be a very complex (many frequencies and associated intensities in the) EM field that happened to interact with the (detector's) surface. To make this any clearer, we'll probably have to start exchanging descriptions of specific thought experiments. Maybe there is something I'm missing. blackcloak (talk) 06:31, 29 July 2009 (UTC)

Maxwell's equations for EM waves do not predict any quantization, but Maxwell's equations do not describe our universe. That's why we figured out the more accurate theory of quantum electrodynamics. This more-accurate theory does predict that monochromatic light has quantized energy, independent of any light-matter interaction. :-) --Steve (talk) 04:30, 29 July 2009 (UTC)

The only reason for mentioning Maxwell's equation in this context is that the article covers the subject in some detail. I did not go back to check if your caution about limitations of Maxwell's equation is in the text. blackcloak (talk) 06:31, 29 July 2009 (UTC)

I have no intention of discussing thought experiments. You seem to believe that the quantization comes from the matter and not the light. That was also Planck's belief, but then Einstein figured it out and physics moved on, and that was 100 years ago. The understanding of light in modern physics is something that's been understood for decades and withstood extremely stringent and specific experimental tests...see Precision tests of QED for example. This goes way beyond the photoelectric effect. :-)

Your reference was to a section in a book dealing with the photoelectric effect. That is what I was responding to. Now you send me elsewhere. I assume you've learned a simple dismissal is not going to work. Your response was a reincantation of dogma, not a response to content. If you think you actually understand an experiment that shows the EM field is itself quantized, perhaps you'd like to take a crack at explaining it. No wiki article I've read has attempted this. blackcloak (talk) 04:05, 30 July 2009 (UTC)

I don't think the article as written implies that Maxwell's equations are exact laws of the universe. But if you disagree, by all means we can make it clearer. :-)

Anyway, regardless of your assessment, the quantization of light energy is universally accepted by physicists, which means it's good enough for wikipedia. See WP:RS. :-) --Steve (talk) 07:43, 29 July 2009 (UTC)

No argument here. That is, when it comes to the part about wikipedia. blackcloak (talk) 04:05, 30 July 2009 (UTC)

The ideas of light being quantized and light being a wave are both accepted, but they don't mix very well. That's "duality". I changed that statement after a source was added, since the source didn't support the idea of the wave being quantized. Dicklyon (talk) 14:33, 29 July 2009 (UTC)

They mix perfectly well as described by quantum electrodynamics. But your edit is fine. --Steve (talk) 15:03, 29 July 2009 (UTC)

Should I be assuming that you think your benediction should be good enough for me? blackcloak (talk) 04:05, 30 July 2009 (UTC)

Someone who appreciates consistency? And then does something about it? Am I to believe my eyes? blackcloak (talk) 04:05, 30 July 2009 (UTC)

One does what one can. I personally think the idea of the EM "field" being "quantized" is misleading, or misleadingly stated, frequently. I'm pretty familiar with QED (though I'm not really a physicist), and I don't think it says anything to suggest that the "wave" is quantized; rather, it talks about wave functions as amplitudes, and quantized observables, which is fine. In any case, adding a source to a contested statement is always good, as it makes it more clear how to fix the statement to agree with the source. Dicklyon (talk) 04:39, 30 July 2009 (UTC)

The original statement -- that the energy of the wave is quantized -- was correct; consult any textbook on quantum field theory. The new version is almost meaningless. But you are right that the original needs a better reference than the one that was provided. Jim E. Black (talk) 07:42, 21 October 2009 (UTC)

[edit] Gravity

I am sure if any one says "black hole" the next thing that is thought of is how gravity effects EMR. But, there are many properties of EMR which are related to gravity. For example, pair production, gravitational red/blue shift of EMR, bending of light near a star. There may be more, but I will let the good people who can write clear articles write them. I just hope someone will make a section on these gravitational properties of EMR. —Preceding unsigned comment added by 76.186.80.92 (talk) 13:45, 13 August 2009 (UTC)

[edit] "EM radiation carries energy and momentum that may be imparted to matter with which it interacts."

The last line of the intro. paragraph:

"EM radiation carries energy and momentum that may be imparted to matter with which it interacts."

...needs some background information and explanation. It seems like it belongs in the middle of a paragraph in the middle of a chapter in a great book on the subject of Electromagnetic Radiation! 207.199.220.113 (talk) 16:45, 13 September 2009 (UTC)

[edit] Question

Is there any type of energy RESISTANT to Electro magnetic radiation? —Preceding unsigned comment added by 67.176.66.143 (talk) 00:26, 12 October 2009 (UTC)

[edit] Mass Question

... highly confusing. EM article includes visible light; the visible light article defines light as "packets" of EM or "photons"; the photon article defines photons as massless. Back to his EM article: "EM radiation carries energy and momentum", since momentum = mass*velocity - I take issue with *something*. I dont know what yet, but definitely something :D (71.58.235.30 (talk) 13:41, 18 November 2009 (UTC))

Momentum = mass*velocity only holds for massive bodies, and only in the framework of Newtonian mechanics. Formally, momentum is the Noether charge of translational invariance (see relativistic momentum). This notion can also be applied to quantum fields and massless particles. — Tobias Bergemann (talk) 17:42, 18 November 2009 (UTC)

[edit] Experiments Section or New Page

There should be a page or a section involving experiments the electromagnetic radiation. Please people look up.

[edit] Electric vs Magnetic Phasing

The opening para. says "Electromagnetic radiation....consists of electric and magnetic field components which oscillate in phase." WHAT! This suggests they are in phase with each other. Aren't they actually 90° out of phase with each other?'

Electric field strength is proportional to the rate of change of the magnetic field. SO MAX magnetic field rate of change is at it's ZERO crossing = MAX Electric field. THEREFORE, 90° out of phase?. Please confirm and/or rewrite this paragraph! If I'm correct the diagrams in article are wrong as well.

"ubiquitous"? Why the'big' words? Not needed. Makes it harder to understand. --220.101.28.25 (talk) 20:40, 24 November 2009 (UTC)