# EM - Light?

1. Jan 22, 2006

### beta3

Hi

Can anyone tell me the differences between normal light (waves of the EM spectrum, i.e. visible light, gamma ray, radio waves, etc.) and magnetic field?
I know that both, light and magnetic fields, consist of photons and i know that magnetic and electric fields are perpendicular to each other.
And I know that Maxwell's equations describe both, normal light and em-fields, too.

But what i can't understand is why one time photons are perceived as light and always go straight out (we are ignoring gravity and there are no obstacles in the way which may cause diffraction and refraction effects, light means here full em-spectrum) and another time photons behave like a magnetic field and go from north pole to south pole and transmit magnetic and electrostatic forces.

And I've got another little question:
How does the spin of the photon affect a magnetic field?
For normal light, one can interpret the spin as the polarization AFAIK.

2. Jan 22, 2006

### pseudovector

I think that by "Magnetic field" you actually mean "static magnetic field" as opposed to the light's oscillating magnetic field.
The main difference between them is that a static magnetic field has very small and insignificant effect on matter (excluding a few metals), whereas a changing magnetic (both in time and space) can couse a variety of phenomena such as inductance.
As for your second paragraph, I think you're referring to the difference between to models of light: wave and particle. The light does behave as both, but the lower the frequency (slower change in magentic field) the wavelike nature of the light becomes more dominant, and the higher the frequency the particle nature of the light becomes more dominant.

3. Feb 7, 2006

### woodsy2k

Wave-Partical duality is an interesting aspect of learning about EM waves. The problem is that there is solid evidence for both cases ie that light is either packets of energy (photons) as they give rise to the photo electric effect, and their trajectory can be changed by very strong gravitational fields, such as black holes. Or as Em waves due to experiments like the double slit experiment.

As for your later question, i was not aware that the spin of a photon affects the magnetic field at all... The spin of photons allows them to "Hand Hold" so that laser action can take place. If there is another property that spin determins, im all ears!

As posted above, i think you might have confused the idea that light infact has a small oscillating magnetic field normal to the direction of propagation. One has to remember that it is the electric field that determins the polarisation of light. (atleast for a linear polarisation, im not sure about spherical or eliptical polarisations)

4. Feb 7, 2006

### Claude Bile

Magnetic fields do not consist of photons.
Not in the general case, but they are indeed perpendicular in a vacuum.
Note that visible light IS an Em-field, albeit a time-varying one.
Diffraction occurs even in the absence of obstacles, you can't omit diffraction from any complete analysis of light.
This sounds like a reference to virtual photons, not real photons.
As far as I know, this spin of a photon is 1. You cannot interpret spin as polarisation, polarisation refers to the orientation of the electric field, not its angular momentum.

Claude.

5. Feb 8, 2006

### beta3

Magnetic fields do only exist if electric fields exist, hence both a existing at the same time and are the same wave (hence the word electromagnetic spectrum)

hm, are you saying all magnetic and electric fields consist of virtual photons and that only light consists of "real" photons?

yes, a photon's spin is indeed 1 and only 1, but it can be - or +1 (or one can say spin up or down), hence we have right-handed and left-handed polarization of the EM-Waves
or do I understand this wrong?

6. Feb 9, 2006

### Claude Bile

Static magnetic fields can exist in the absence of an electric field.
Electromagnetic force is transmitted via virtual photons. EM waves consist of real photons.
There are an infinte number of polarisation states, so it doesn't seem feasible to relate polarisation to photon spin, which has only 2 possible states. You could certainly use the right-hand corkscrew/left-hand corkscrew as an analogy for photon spin (as it would seem the best way to visualise such a conceprt), but as for any rigid connection, I am doubtful.

Since my knowledge of such topics is a little fuzzy, maybe I will leave it to one of the other members to confirm or refute the connection between photon spin and polarisation.

Claude.