# Simple photon question

1. Aug 2, 2004

### mee

why is light refereed to as the electromagnetic spectrum when light carries no charge and is unaffected by magnets?

2. Aug 2, 2004

### marlon

Well the entire EM-spectrum is just a list of light of different frequencies. This means light at different energy scales. For example the visible part of the EM-spectrum starts with the lowest energy or frequency a human eye can see : red is low energy. When the energy gets higher, the colour of light evolves to blue. Off course there is much more in the EM-spectrum then just the visible part. Low energies are radio-waves and IR. Above blue there is for example x-rays and UV-light

3. Aug 2, 2004

### marlon

What do you mean with the fact that light carries no charge. That is very true, but what does it have to do with the spectrum. Generally a spectrum is just a list off different possible energy-values of one fysical system.

regards
marlon

4. Aug 2, 2004

### ZapperZ

Staff Emeritus
It is historically called "electromagnetic" because from Maxwell equation, the description of light contains two oscillating components: an electric field vector and a magnetic field vector, thus the "electro" and "magnetic" parts.

Zz.

5. Aug 3, 2004

### humanino

light IS affected by magnets !

6. Aug 3, 2004

### vanesch

Staff Emeritus
No.

Well.

If you want to nitpick, I guess it is, as photon-photon interactions.

But for all practical purposes, it isn't.

cheers,
Patrick.

7. Aug 3, 2004

### mee

If anyone has the time, perhaps one could explain to what these vectors refer? Thank you very much if you take the time to explain this to me.

8. Aug 3, 2004

### zefram_c

It would take a long time to explain what the electric field and the magnetic field vectors are if you haven't encountered them yet - a textbook on classical electrodynamics would be a far better solution for you. Of course, we'll help along with any questions you may have. If you do know basic E&M, perhaps you can rephrase your question to clarify what you need explained?

9. Aug 3, 2004

### what_are_electrons

"Simple" photon question - pt2

Mee's photon question is the same question I posed to another forum very recently.
Perhaps the question should be rephrased to:

"What is the experimental evidence / proof that light has either electric or magnetic properties?"

Maxwell's equations do not prove that light (photons or quanta) are made of electromagnetic radiation/disturbance/waves/.... Maxwell's equations were derived from experimental evidence (from Faraday, Lorentz and Gauss) involving solid state materials with electric currents.

Maxwell only suggested that light was made of EM.
Hertz took his suggestion and made an antenna.

Microwaves (one frequency of photons) are routinely used to transfer momentum to electrons in synchrotrons, but this is not proof that photons have electric or magnetic properties.

Antenna emit radio waves (another frequency), but this does not prove that radio waves have electric or magnetic properties.

Photon entanglement is not a proof of E or M properties.

Electrons absorb and emit photons just like an antenna, but that is not proof.

Electrons are influenced by E and M fields, which shows that electrons have both E and M fields, but electrons are not photons.

Protons can also absorb and emit photons. This is shown by NMR.

Magnetic fields and electric fields are known to produce a change in the energy (frequency) of the photons that excited electrons emit as they decay to ground state, but that is not proof that photons are affected by E or M fields. The change caused by the magnet or electric field is due to work (energy added or lost) done to the electron before or after it emits or absorbs the photons. There is no proof that the magnetic or electric fields have influenced the photons once they are freed from the electrons.

So, the question remains:
"What is the experimental evidence / proof that light has either electric or magnetic properties?"

10. Aug 3, 2004

### Danut Argintaru

The electron has electric and magnetic properties because it produces an electric and
a magnetic field (when it is moving) and because it is the subject of the electric and magnetic forces when it is placed in exterior electric and magnetic fields. The photons are the quanta of the E.M. fields, so they are the E.M. fields.

11. Aug 4, 2004

### ZapperZ

Staff Emeritus
Visit a particle accelerator. Look at the RF accelerating "cells" that are used in both the photoinjector and the linac. The particles (typically electrons) are accelerated purely via either standing wave or travelling wave RF fields (which is of course, an EM wave). Without the presence of the E-field in the correct geometry, this would not have been possible. In fact, practically all calculations and design of photoinjectors and accelerating structures for accelerators use classical E&M theory.

Secondly, the fact that Maxwell Equations were derived from experimental observations is in itself, an experiment proof of the validity of the idea that EM radiation contains both E and B field. I mean, what stronger empiricial evidence can we have than having theory that is consistent with practically ALL of classical E&M observation?

Thirdly, visit any synchrotron center and look at experiments that make use of the intense light coming off one of the insertion device. The polarization of that light (the evolution of the E-field as defined in Maxwell Eqn) plays a significant role in experiment ranging from optical conductivity to photoemission, to x-ray diffraction! If light has no E and B field, it will then be doing something completely mysterious in experiments such as angle-resolved photoemission, and the semiconductors in your modern electronics should not work (the band structures of most widely-used semiconductors were experimentally confirmed using photoemission measurements).

Zz.

12. Aug 4, 2004

### Nereid

Staff Emeritus
It may be an extreme case, but magnetic fields do affect photons, http://www.journals.uchicago.edu/cgi-bin/embpcgi.pl/cgi-bin/res-page.epl?objid=374334 [Broken], which should be observable in magnetar atmospheres.

Last edited by a moderator: May 1, 2017
13. Aug 4, 2004

### ZapperZ

Staff Emeritus
Well, do you think it is actually fair to bring in exotica such as vacuum polarization/fluctuations, etc., simply to show this? If we have to invoke such a picture, then we shouldn't be talking about an EM wave (and classical field) in the first place, since in QFT, there are no such thing as a classical field. So the original question then (how "electromagnetic" radiation got its name) becomes moot and irrelevant.

Can we at least agree upon the idea that CLASSICAL E&M indicates no apparent interaction of EM-radiation with magnetic fields?

Zz.

Last edited by a moderator: May 1, 2017
14. Aug 4, 2004

### what_are_electrons

We both know that light (microwaves in this case) can transfer momentum to electrons, but that is not proof that light has EM properties. Tell me - how is it that classical EM theory can be applied to a quantum particle when classical defines a band or a sea of electrons with all states filled?

Sorry, but the experimental observations used by Maxwell were achieved using solid state materials. There is no EM radiation (photons of light) within a solid. It seems that we need to discern EM fields generated by currents within solid state and EM radiation which is outside the solid state. Being consistent with a theory is not proof. I need experimental evidence.

I agree that the magnets cause the electrons to emit light by forcing the electrons to change energy state, but that does not prove that light has EM properties. It is not necessary to use polarized light to produce those effects so I don't see your point. I agree that light is indeed absorbed by electrons and/or parts of the core, but that does not prove that light has EM properties.

My objectives in this topic are to:
(1) begin a serious effort to better define the inherent properties of light within a classical view and to combine that with a quantum field theory if possible and needed
(2) in conjunction with the objective of (1) is a need to define the true nature of the electron within a classical framework, not the statistical probabilities now using in QM

It's time to bring physics back to reality. That is one reason for the quote I use from Einstein.

15. Aug 4, 2004

### Norman

Huh???? I am not completely familiar with the setup used in these experiments, but fields can penetrate matter. Let us not be foolish here.

16. Aug 4, 2004

### ZapperZ

Staff Emeritus
What does a "filled state" have anything to do with momentum transfer from light to electrons? Secondly, how do you think this momentum transfer occur? Since light has no mass, the ONLY means of transfer is via an interaction of the electron with an electric field. Since there IS an energy transfer, it cannot be just from a magnetic field, so there has to be an electric field present. It is this axial electric field within a photoinjector cell and the linac that is the mechanism of acceleration for these electrons. And the fact that the GEOMETRY of the system and the direction of the E-field in the RF system MATTERS on whether we have an acceleration or not is clearly a proof. And keep in mind that these are FREE electrons. They are not confined to any solid and therefore have no "band or sea of filled states"!

Er.. hello? Look up an area of condensed matter physics (of which solid state physics is a part of) called optical conductivity. In it, you will see propagation of light/EM radiation/photons through solids. Experimental techniques such as Raman spectroscopy and FTIR all use this phenomena! It is one of the techniques we use in condensed matter to study the phonon structure of solids!

I was hoping you would at least try to figure out what "angle-resolved photoemission" is before you replied to me. It would at least give you an indicaton why the polarization of the photon matters in such experiments. I guess I was wrong. My avatar shows the photoelectron intensity of electrons moving in a particular direction in a crystal. It was done with a plane-polarized light having the SAME direction as the momentum of the electrons. If I use the identical light, but with the polarization rotated by 90 degrees (but keeping the same Poynting vector direction), I get almost nothing! Both classical and quantum theories clearly show that the most effective transfer of energy and momentum between an electric field and electrons in solids is when they the E-field and the electronic momentum are parallel to each other. This is the fundamental ingredient of the description of a photoemission phenomena - the coupling between the E-field vector of the light incident light with the crystal lattice. This is such a well-known phenomena that we USE it to study materials with - thus my statetement that your semiconductors in your modern electronics would not work if what we learned from photoemission results are wrong!

Then it appears that you are trying to bring up unverified ideas of your own. This then belongs in the Theory Development section. BTW, have you ever considered that the reason why you do not "see" this is because you haven't studied it, nor done it? I'd say my advice to you to visit an accelerator and synchrotron facilities was timely. I have or am working at both, and I have no problem "seeing" the principle of physics at work everyday.

Zz.

17. Aug 4, 2004

### Nereid

Staff Emeritus
One person's 'exotica' is another's everyday fare?
D'accord. However, the original question was done and dusted by post#4 (yours, I think); as often happens in threads here, it started to move onto something else.
Yes.

18. Aug 4, 2004

### what_are_electrons

First things first.
I've been using mono-XPS (HP, VG, SSI, PHI) and AES for the past 20 years, so am reasonably familiar with AR-XPS, but not via a synchrotron.

Please clarify your statement about transfer. Does light transfer momentum or energy? Does light transfer its momentum (or energy) via electric field only or both electric and magnetic fields? Which field dominants the transfer?

I am trying to resolve how light transfers momentum or energy. That is part of the point of these discussions. You mentioned energy transfer. Do you really mean energy transfer or momentum transfer? Once the photon transfers that momentum or energy, where does the photon go? Back in time?

To observe, measure, record or prove the existence of the E or M properties of light, they must be tested in a vacuum, not within condensed matter which is a completely different medium that is filled with particles that are well proven to have electric and magnetic properties. So, let's drop solid state phenomena from these discussions.

I would greatly appreciate it if you would answer a few of the following questions:

(1) Have you ever read about any experiments done in UHV that pass any form of light between two poles of a biased electric field or a biased magnetic field, pulsing or static, that has caused the deflection of the light or caused a change in wavelength (energy)?

(2) What is your basis for saying light has no mass?

(3) What is your working definition of mass?

(4) Based on classical EM or QFT, is the magnetic field vector of light larger, smaller or the same intensity as its electric field vector? Why?

(5) What is the proof that an electron (free or bound) has a Mechanical Mass at its center?

(6) Do the magnetic field and electric field vectors of the electron have the same, larger or smaller intensities? Why?

(7) Are the electric and magnetic field vectors really equal to zero at the nodal point of an EM wave?

(8) Not sure what you mean by Optical Conductivity? Is this photoconductivity?

(9) Why do relativistic electrons only emit light when turned?

I am modestly familiar with the polarization of the synch beam. The beam is known to be "linear" polarized in the orbit plane and "elliptical" polarized outside the plane assuming the beam is relativistic.

When we talk about linear polarization, we are talking about light that has passed through the electron and atomic core structure of a solid state (condensed matter) crystal that produces the observed change (linear polarization) in the light. A crystal is a highly complex set of electric and magnetic fields which are not well defined. (I say this because QM can only do a "good" ab initio projection of hydrogen which has only a single electron and single proton.)

The questions that come to mind in the production of linear polarized light are: What happened in the atom to produce that change? Did one or more of the electrons produce that change? Was it just the electric field of a single electron or was it a set of electrons that produced the change? What is the shape of the electric and magnetic fields of the electron that presumeably absorbed part of the original beam and passed the emitted beam of light that is now "linear" polarized? Does the EM field of a single electron have a peculiar shape that causes light to be emitted in mainly one direction when the excited state is decaying and emitting the photon?

I'm saying that we make good use of polarization in many different spectroscopies, but unfortunately, we still don't know what is going on because we have used solid state matter to produce light polarization.

Again, we must not use solid state physics to study light. I still need an example of experimental proof that shows that light has E and/or M properties.

Sorry, but optical birefrigence is a solid state phenomena and does not constitute proof that light has E or M properties.

19. Aug 4, 2004

### meteor

the Faraday effect rotates the plane of polarization of plane polarized light. This rotation is achieved applying a magnetic field to light

20. Aug 4, 2004

### what_are_electrons

To observe the Faraday effect in Magnetic Circular Dichroism, the sample, normally a solution, sits inside of the static magnetic field of a strong fixed magnet. In this case, the EM of the electrons in the atoms are influenced by the magnetic field of the fixed magnet. The electrons then absorb light and decay. The Zeeman effect is a similar effect and it too is produced by surrounding a material with a magnet while the atom is photo-excited or vaporized ...