Do electromagnetic waves extend backwards?

Thread Starter

schoolie

Joined Oct 5, 2011
5
How long is an electromagnetic wave? Can it be of any length depending on the source? or is it some integral/even multiple of the wavelength? Also, can an electromagnetic wave have varying magnitudes of electric and magnetic fields (at varying distances on the wave) depending on varying frequency of oscillations of the source? and how does it terminate?

(Note that I haven't read the chapters on transverse waves yet, but I think I have an idea of it)
 

Wendy

Joined Mar 24, 2008
22,138
The length of a wave in free space is simplicity itself, it is

speed of light (186000 miles / sec) ÷ frequency (cycles / second)

You are left with 186000 miles / cycles, the seconds cancel and cycles is basically a dimensionless number.

When building antennas a resonant wire is basically ½ of that X 96%
 

davebee

Joined Oct 22, 2008
540
Bill gave you the formula for the wavelength of the wave, but are you actually asking what the overall physical length of a wave?

"Can it be of any length...?"

The wave has the upper limit of length as how long the energy source has existed times the speed of light.

Does a wave have a lower length limit? In other words, how long is a photon?

I thought this would have an easy answer, but as far as I can tell, nobody knows.

Physics can tell you things like the energy and spin and wavelength of a photon, but can't say anything about its physical size. As far as I can tell (and I'm not an expert in this; maybe someone will correct me) there simply aren't any experiments or theories in existance that give a length for a photon.
 

Wendy

Joined Mar 24, 2008
22,138
I've often wondered if our 50 and 60 Hz hum will be noticed. Space junk (such as dust clouds) tends to absorb a lot of frequencies. Another word for the question (I think) is the spectrum, which is quite wide.
 

studiot

Joined Nov 9, 2007
4,998
I've always wanted to do this so here goes

A long time ago in a galaxy far far away

a star was born

The star started emitting EM radiation.

This radiation spread outward continuously for 4 billion years

Then the star died so the radiation stopped.

So travelling through space we have a a specimen of EM radiation 4,000 million light years long.

go well
 

nsaspook

Joined Aug 27, 2009
7,181
Physics can tell you things like the energy and spin and wavelength of a photon, but can't say anything about its physical size. As far as I can tell (and I'm not an expert in this; maybe someone will correct me) there simply aren't any experiments or theories in existance that give a length for a photon.

The string theory (no point particles) size of a photon is about 10e-33 to 10e-44 meters long.
http://library.thinkquest.org/27930/stringtheory2.htm
http://www.speed-light.info/string_theory_01.htm

What this has to do with reality is another question.
http://universe-review.ca/F15-particle.htm

 

Thread Starter

schoolie

Joined Oct 5, 2011
5
Thanks, everyone.

May be I should put this back until much later. The reason I asked this was I didn't understand how polarization worked. The textbook explained the polarization of "an electromagnetic wave" (singular), so I was doubtful whether a single electromagnetic wave was polarized differently at different lengths of the same wave (is that possible?). So does polarization apply for single as well as multiple waves (is there a difference)?

Anyway is there any truth in the following representation of an electromagnetic wave:
http://www.phys.hawaii.edu/~teb/java/ntnujava/emWave/emWave.html
(Note that the wave at the 'tip' travels at a different speed from the waves following it; reset the player if you didn't notice it). Never knew visualising it was so difficult but I have to do it first before moving to induction.

@nsaspook : How can photons have dimensions from 10e-33 to 10e-44 meters, which is less than the wavelength of most light (assuming, by "photon" you meant the entire wave)
http://forum.allaboutcircuits.com/member.php?u=59115
 

Wendy

Joined Mar 24, 2008
22,138
The reason the waves are polarized is the antenna itself. There are lots of examples where light and radio starts off polarized. Lasers and LEDs, for example, are typically that way, which can be proved with simple polarized lens, such as is used in the glasses given away in movie theaters.

There are antenna designs where more than one polarization exist, but it is more trouble than it is worth. Haven't you ever wondered why a typical antenna is straight up and down? It matches the polarization of the transmitter.
 

nsaspook

Joined Aug 27, 2009
7,181
Thanks, everyone.


@nsaspook : How can photons have dimensions from 10e-33 to 10e-44 meters, which is less than the wavelength of most light (assuming, by "photon" you meant the entire wave)

Photons have no size if they are zero-dimensional particles (energy packets). They don't expand or contract as the frequency of it's oscillation changes. When treated as a wave it has a wavelength and amplitudes of the EM fields of the point particle as it moves in space.
http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec13.html
In string theory particles are modeled as single dimension 'strings'.

 
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