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How can light be called electromagnetic if it doesn't appear to be electric nor magnetic?

If I go out to the sunlight, magnets aren't affected (or don't seem to be). And there is no transfer of electric charge/electrons (as there is in AC/DC current in space).

In particular, the photons (which light is supposed to be composed of) have no electric charge (nor do they have magnetic charge).

I'm looking for an explanation that can be appreciated by the average non-physicist Joe.

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It's like the difference between air pressure and sound. Sound is just rapidly changing air pressure. –  Mike Dunlavey May 27 '13 at 21:48

12 Answers 12

up vote 28 down vote accepted

Light is an oscillating electric and magnetic field, so it is electrical and magnetic.

Later: re the edit to your question, I think there are two issues. Firstly the interaction with electric charge and secondly the interaction with magnets.

Light does not carry any charge itself, so it does not attract or repel charged particles like electrons. Instead light is an oscillating electric and magnetic field. If you take an electron and put it in a static electric field (e.g. around a Van der Graaff Generator) then the electron feels a force due to the field and will move. This happens when an electron interacts with a light wave, but because the light wave is an oscillating field the electron moves to and fro and there is no net motion. If you could watch an electron as light passes by you'd see it start oscillating to and fro, but it's net position wouldn't change.

This is exactly what happens in your TV aerial. The light (i.e. radio frequency EM) causes electrons in the TV aerial to oscillate and this oscillation generates an oscillating electric current. The voltage this generates is amplified by your TV. At the TV transmitter the same happens in reverse: an oscillating voltage is applied to the TV transmitter, the electrons oscillate in response and the oscillation generates an electromagnetic wave. So the process is oscillating electrons -> light -> oscillating electrons.

I'm not entirely sure what you mean by there is no transfer of electric charge/electrons (as there is in AC/DC current in space). If the above doesn't satisfactorily explain what's going on maybe you could expand on your question.

And finally on to the interaction with magnets.

The big difference between electric and magnetic fields is that (as far as we know) there are no isolated magnetic charges. If there were isolated magnetic charges e.g. if you could watch a magnetic monopole as a light wave passed by then you'd see similar behaviour to an electron. But there aren't, so you don't.

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If I were the OP, I would want more then a restatement of the proposition I am questioning. –  antony.trupe Oct 26 '12 at 2:18
    
What would you want if you weren't the OP? Since you aren't, then that is more pertinent. :) –  Kaz Oct 26 '12 at 3:07
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This is of course completely correct, but I don't really think it answers the question, which is implicitly asking why sunlight doesn't appear to be electric or magnetic, particularly in form of the more everyday forms of electricity and magnetism. –  Jefromi Oct 26 '12 at 4:20
    
(v2) ...If you could watch an electron as light passes by you'd see it start oscillating to and fro, but it's net position wouldn't change... Mhm, so eihter you model the things as a perfectly smeared out plane wave or as a package which can "pass by". I'm not sure then if one can simple argue with the classical oscillating potential for the charged electron, resulting in no net movement. And even an oscilating electron would imply a changing electric field and then more light, right? Now things get blurry. In any case, at one point the QED tree graph must kick in. –  NikolajK Oct 26 '12 at 7:59
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I'm sensitive to an explanation that can be appreciated by the average non-physicist Joe. There's a limit to what can be accomplished given this limitation. –  John Rennie Oct 26 '12 at 8:04

How can light be called electromagnetic if it doesn't appear to be electric nor magnetic??

But light does appear to be electric and magnetic in nature. For example:

Photovoltaic effect:

The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light.

Focus: Measuring the Magnetism of Light

Now two groups have independently demonstrated that a tiny, metallic probe will interact strongly with the magnetic field of light waves trapped in a sort of semiconductor “box.”

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How can light be called electromagnetic if it doesn't appear to be electric nor magnetic?

According to the theory of Electricity and Magnetism, charged particles which are stationary are "electric", charged particles which move at a constant velocity are "magnetic", and charged particles which accelerates will emit "electro-magnetic radiation" which travels at the speed of light.

Charged particles can't interact instantaneously, but rather there is a field of energy which mediates their interaction. This field of energy is what we call "the electromagnetic field'.

In other words, "light" is the transportation of energy from one part of the electromagnetic field to another, and it facilitates the interaction between electric and magnetic objects, but is neither electric nor magnetic itself.

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Sound is a mechanical vibration, but you can only sometimes observe it making things vibrate. Sometimes the vibrations are too small, or too fast. The same kind of thing is true of light.

Visible light has a wavelength from about 400-700nm, far smaller than anything you can discern unaided. This corresponds to frequencies of about 4-7*10^14 Hz, far faster than anything you can perceive.

So yes, at a very small and fast scale, sunlight is composed of electromagnetic waves (oscillating electric and magnetic fields), and they do have elecromagnetic effects on things - they're just not usually visible effects.

But for example, radio is also electromagnetic radiation (with much longer wavelength and lower frequency than visible light), and it has electrical effects, so an antenna can convert between radio waves and oscillating electrical currents.

Magnetic effects are a bit tougher to observe. Perhaps the most commonly known one is the Faraday effect - magnetic fields can rotate the polarization of light.

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This image (taken from Wikipedia) demonstrates what an electromagnetic wave looks like.

img

Changing electric fields induce a magnetic field (this is how electromagnets work), and changing magnetic fields induce an electric field (this is how the charger on your electric toothbrush works). The result is that if one oscillates, so will the other, and they will continually induce each other.

I hope that at least gives an intuitive explanation for it (even if some of what I said is not 100% technically correct).

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That's a common textbook illustration, now animated and on the web, but gives a misleading impression of what EM waves are like. –  DarenW Jan 29 '13 at 21:00
    
@BrandonEnright - can you please elaborate on why this animation is wrong? (Every credible book says that electromagnetic waves are a product of varying electric and magnetic fields which induce each other). –  grjj3 May 26 at 20:17
    
@DarenW - can you please elaborate on why this animation is wrong? (Every credible book says that electromagnetic waves are a product of varying electric and magnetic fields which induce each other). –  grjj3 May 27 at 20:26
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I also would like to know. I took this image directly off the Wikipedia article, and it is still there today. The text of this answer is, of course, my own. Is your issue that this explanation doesn't account for quantum wave-particle effects. –  asmeurer May 27 at 20:44

Light is a propagating electromagnetic field disturbance. If we ignore certain obscure quantum effects, light is not bent by electric or magnetic fields because it does not carry charge particles. The field disturbances simply superimpose onto whatever electric and magnetic fields are permeating the traversed space.

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As others have pointed out, light is actually both magnetic and electrical. It is part of the electromagnetic spectrum, which has everything from invisible light such as gamma rays, infra red and xrays to the visible light that you speak of.

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To illustrate the magnetic effect of e.m. wave on matter my favorite example is to talk about the microwave.

The microwave is a lamp but emits invisible light (lower frequencies). Every polarized molecule will oscillate if subjected to this field.

The result is that the molecule (i.e. water) are oscillating (vibrating) and therefore the temperature increases (the temperature measures the average "speed" of molecules).

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+ Close. The electric field does not directly vibrate the water molecules. It causes impurity ions that carry charge, like sodium and chloride, to move (i.e. have electric current) in the water, just like charges move in an antenna. Since that current encounters resistance, it generates heat, that cooks the food. –  Mike Dunlavey May 27 '13 at 21:56

See this wonderful youtube video to see the electric effects of light using house hold items

http://www.youtube.com/watch?v=WO38qVDGgqw

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This is a great demonstration of the photoelectric effect, but the photoelectric effect doesn't exactly demonstrate that light is electromagnetic. It could just be some other kind of particle that's capable of knocking electrons loose. –  Jefromi Oct 26 '12 at 4:32

Here's one of my favorite explanations. While others here have done a good job, I like this for it's bit of history for the discovery of the fact that light is a electromagnetic wave. It takes a moment, but stick with it.

http://www.youtube.com/watch?v=NMgcX8UNIGY&list=PLED25F943F8D6081C&index=3&feature=plcp

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it's usually preferred to include a description or summary of the contents of a link, rather than simply the link. Our concern is that link may no longer exist in the future. –  Colin McFaul Oct 26 '12 at 19:24
    
Understood. Will do better. –  MartinK Oct 26 '12 at 20:49

Light is called an 'electromagnetic wave' for historical reasons* in the following sense: It turned out that the effects of visible light and other radiation can be calculated using Maxwell's equations, which are also used to model the behaviour of electrically charged particles. This was an instant of a successful unification and it hasn't been dismissed since. Nine answer and the word "Maxwell" has not been used yet! Please see also the following wikipedia article, which contains a section First to propose that light is an electromagnetic wave.

(*That bold sentence is essentially a tautology: People name things, and so names are not independed from previous experience. At least both the Descriptivist theory of names by Russel et. al. and the more modern Causal theory of reference by Saul Kripke exibit this feature.)

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As light is produced by the acceleration of charged particles & from law of electromagnetism that states that: an accelerated charge produce electromagnetic wave,light is an electromagnetic wave. Actually light is the transfer of energy from one part of electromagnetic field to other. Everyone knows how the electromagnetic wave looks like(see answer by asmesure). As electric and magnetic field are perpendicular to each other they behave as crossfield but it does not mean that the charged particles feels no net force.They do feel some force, that is why charged particles oscillates by passage of electromagnetic wave.But their amplitude of oscillation is very small. So we don't see or feel charged particles affected by light though its electromagnetic. However light facilitates the interaction between electric and magnetic objects.?

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Particles do feel a force which is why charged particles oscillate with a passing EM wave. –  Brandon Enright Jan 27 at 16:46

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