[namaan (OP)]

Hey folks,

I'm curious if there's an experimental analog of EM waves to the sine wave graphs that can be produced through measuring sound waves. Of course the frequency of an EM wave can be measured, but has the actual 'trajectory/path' that EM waves make been graphed onto paper? In other words, are the diagrams (i.e. http://en.wikipedia.org/wiki/File:Onde_electromagnetique.svg) of the graph of an EM wave educated assumptions derived from the general behavior of waves, or are they experimentally derived?

BTW, I'm not sure what the policy on posting links to other sites is, but I assumed Wikipedia isn't an issue. Do let me know otherwise.

[Geezer]

Hi Namaan:

Good question, and I think your use of the Wiki link is quite appropriate because you have explained your question well.

I know that EM waves do interfere with each other and these effects can be observed by measuring field strengths using an antenna. It's relativey easy to do this with low frequency radio frequencies because the wavelengths are long, so the distances between the maxima and minima signal strengths can be quite large.

Also, the relationship between antenna lengths (and polarization) and signal frequency reinforces the idea of wave like behaviour.

However, these examples are not really direct measurements, although they do provide strong evidence. Let's see if someone knows of some direct measurement techniques.

[yor_on]

No, as far as I know wikipedia is cool, there might be 'biased answers' there, but my guess is that we will learn about it soon enough if so There is no guarantee that an answer is unbiased anyway, even the main stream ones can be it in retrospect. It's all about experimental evidence, and even that can change when something new gets known. As I see it that is.

[Ron Hughes]

Any graph of an EM wave is a mathematical reproduction of that wave since no one knows what a photon is they can't know the actual path. I will post what I think the photon is under the new theories section.

[JP]

Any graph of an EM wave is a mathematical reproduction of that wave since no one knows what a photon is they can't know the actual path. I will post what I think the photon is under the new theories section.

Thinking about photons is just confusing the issue here.  The photon model is much more complicated to use for trying to model an electromagnetic wave like the poster is acting about.  You could do it, but why bother when the classical model works just as well?  Even if you do want to look at photons, you can measure them.  There are plenty of experiments which demonstrate measuring photons as single "clicks" on a detector.

[JP]

It depends what kind of plot you want to see.  That side-on plot you show is going to be extremely difficult to see if the wave is traveling in empty space because it moves at the speed of light.  Trying to see that nice structure is going to be impossible in practice because any measurement you try to take is going to involve some time, and the speed of light is so fast that it will move in that time and look blurry.

One thing you can do is to put the wave in a box so that it bounces back and forth, creating what is called a standing wave.  This does look like the wave in your picture and there are a variety of ways to take a "picture" of it.  Essentially where the wave peaks are, you have a lot more energy, so you can put something in your box to absorb the energy and therefore take a picture of the basic structure of that sine wave.  In fact, that's basically what this experiment is about.  You can also use a polarizer which is a simple device that only allows waves that are waving in the right direction to go through.  This tells you how the E and B fields are oriented in space.   Those are the two techniques I can think of off the top of my head to come up with a picture like you showed for EM waves. 

[lightarrow]

Hey folks,

I'm curious if there's an experimental analog of EM waves to the sine wave graphs that can be produced through measuring sound waves. Of course the frequency of an EM wave can be measured, but has the actual 'trajectory/path' that EM waves make been graphed onto paper? In other words, are the diagrams (i.e. http://en.wikipedia.org/wiki/File:Onde_electromagnetique.svg) of the graph of an EM wave educated assumptions derived from the general behavior of waves, or are they experimentally derived?

BTW, I'm not sure what the policy on posting links to other sites is, but I assumed Wikipedia isn't an issue. Do let me know otherwise.

The 'trajectory/path' you are talking of is valid *only* in the case of a plane wave. A good approximation can be a very collimated laser beam, in the region occupied by the beam (I say this because a plane wave actually is infinitely extended).
In a more common situation, for example light emitted by a bulb lamp, light doesn't have a defined trajectory, even because it can 'deviate' because of diffraction (in the first years of XX century they used to say that light doesn't have 'nadelstrahlung' = needle-like emission).

[Farsight]

namaan, the wiki image is akin to a snapshot of a whole series of waves:



Like Geezer said, it's experimentally derived. And like lightarrow said, that's a plane-polarized depiction. Maybe I've misunderstood your question, but I'd say the trajectory/path of the EM waves is a straight line. You can perhaps understand what I mean by by looking at a snapshot of oceanic swell waves:



If however you've ever been on a ship you might have leaned on the rail and watched a single swell wave go past. It's like a bump in the ocean, bigger than the ship, barrelling along in whatever direction it's travelling, in a straight line. It causes the surface of the sea to undulate in a sinusoidal fashion as it passes, but it doesn't in itself snake up and down.

[Geezer]

In the visible spectrum EM wavelengths are extremely short, so it would be very difficult to observe peaks and troughs at those frequencies. However, at very low electromagnetic frequencies, the wavelengths are considerable. Interference effects can be observed at those frequencies. I may be wrong, but I think it should also be possible to measure the wave effect directly at low frequencies.

[Farsight]

That's essentially what we do when we see, Geezer!

Can I make an amendment to what I said earlier. There is a slight issue with the typical depiction of an electromagnetic wave, such as the one here:

http://simple.wikipedia.org/wiki/Electromagnetic_wave

It separates the electromagnetic wave into an electric field component and a magnetic field component. That's a little misleading, because there's only one field involved, the electromagnetic field. 

[Geezer]

That's essentially what we do when we see, Geezer!

 

Farsight - your eyeball is incapable of differentiating the peaks and troughs in visible EM. A radio antenna can differentiate those peaks and troughs.

[Farsight]

If it couldn't, I couldn't see.

[Geezer]

Farsight: I'm not sure how you would be able to measure the wavelength of light using your eyes and a ruler.

[acsinuk]

Hi Farsight
The diagram from wiki looks like a 3D helix to me. The electromagnetic helix contains a volume of photon flux [magnoflux] and not particles as implied by the +/-"q" signs which refer to the space medium conducting parameter only. The continuous wave energy should be measured in Watt seconds [Joules] per cubic metre in my view.
CliveS

[Farsight]

acsinuk: there's some helicity to a circularly polarized light wave, in that the electric vector rotates. See http://hyperphysics.phy-astr.gsu.edu/HBASE/phyopt/polclas.html#c3 :



And I've been involved in discussions re photon angular momentum and volume, and why the photon doesn't spread out, but I can't see any electromagnetic helix in a plane-polarized photon I'm afraid.

[yor_on]

JP that was one of the coolest experiments I've seen

And it's mine... Yes it is, Chris, Dave and those others might try to say that they were there first but they weren't.

I can swear to that, I looked around the first thing I did as I arrived, and honestly, there was no one there. So even though you have the honor of discovering it, I must be the second man in the history of the earth to have seen this. From now on, no micro-wave oven is holy for me and my scientific endeavors will find no cease.

[JP]

I've seen it done using silly putty that changes color with temperature as well.  That makes it a bit easier to see the hot spots in the microwave.  Plus, you get to play with silly putty.

[Geezer]

Well, there you go then. Obviously EM radiation is due to waves propagating through something, even though we prefer to think it's nothing.

How could we explain this phenomenon in terms of particles? Would the particles all have to collaborate and do a wave, like people do at baseball games?

[JP]

Geezer, essentially yes.  Classical waves are a special way the photons of light cooperate, known as a coherent state.  http://en.wikipedia.org/wiki/Coherent_state

Photons don't have to be in a coherent state--and indeed a lot of quantum optics deals with states that aren't like classical waves, for example a single photon.

[Geezer]

I should have known better.

Now I'll have to assimilate all this stuff if I want to continue the argument.

Sheesh!