# Light electromagnetic?

1. Apr 19, 2006

### row

i have been wondering that if light as wave is nothing but osscilating electric and magnetic fields then why dont they(gamma rays) deflect in the experiments where electric fields or magnetic fields or both are present?
since both E and B are vectors?

2. Apr 19, 2006

### inha

The em-wave picture is the classical picture. You need quantum electrodynamics to properly describe light. I'm not well versed in it but some members are and I'm sure they can help you out.

3. Apr 19, 2006

### row

thnks inha, actually i had asked my teacher too.he did tell me something about a combined vector form of magnetic and electric field that causes no interference with stray external fields but i still have doubts as to how the 2 combine in the first place.

4. Apr 19, 2006

### vanesch

Staff Emeritus
Your teacher is correct. Because the equations gouverning classical electromagnetism are LINEAR equations, there's no influence of one solution upon another. Remember, linear equations have the following property: if sol1 is a solution to the equations, and sol2 is, then sol1 + sol2 is too.
Now, take "sol1" as the field of the radiation (light, or gamma rays or whatever), and "sol2" the applied B or E field. Well, this means that they can happily exist together, as sol1 + sol2. In other words, there is no MODIFICATION of sol1 because of the presence of sol2.

5. Apr 26, 2006

### Twukwuw

hi row,

I think your question is, why the photon does not deflect, even though it is composed of electromagnetic field.

My idea is,
because the photon carries No charge, hence it will not "feel" the external electric field.

Twukwuw.

6. Apr 26, 2006

### masudr

Again, this is classical EM question; not much to do with quantum mechanics.

An electromagnetic wave is just that: a pair of oscillating electric and magnetic field vectors. The equations they obey is:

$$\vec{\nabla}^2\vec{E} \left( \vec{x} , t \right) = \frac{1}{c^2}\frac{\partial^2}{\partial t^2}\vec{E}\left(\vec{x},t\right)$$

and a similar equation for $\vec{B}$. These equations are derived from Maxwell's equations, and are standard in any undergraduate physics course.

If I may possibly ask you a question, why do you think the electric field should deflect in the presence of external fields? The equations do not say they should.

The things that do deflect are things that have charge as Twukwuw says, since the force law is:

$$\vec{F}=q\left(\vec{E}+\vec{v}\times\vec{B}\right),$$

and EM waves possess no charge.

Last edited: Apr 26, 2006
7. Apr 26, 2006

### Staff: Mentor

You're mixing the classical and quantum pictures of light here.

In the classical picture, light is an electromagnetic wave, and electric and magnetic fields obey the principle of superposition as vanesch described. Therefore, in the classical picture, light cannot be affected by other electric and magnetic fields.

In the quantum picture, light consists of photons which have direct (first-order) interaction only with charged particles like electrons. Electric and magnetic fields are the collective effect of many many virtual photons. Therefore, also in the quantum picture to first order, light cannot be affected by electric and magnetic fields.

Nevertheless, in the quantum picture it is possible for photons to interact indirectly via higher-order processes that involve the creation and annihilation of virtual particle-antiparticle pairs. This photon-photon scattering has been observed in high-energy accelerator experiments. The effect is much weaker than ordinary interaction of photons with charged particles.

It should be possible for real photons to interact weakly with virtual ones (in electromagnetic fields) by the same mechanism. I think this would fall in the area of non-linear optics, which I'm not an expert in, but which is a well-developed field of study.

Last edited: Apr 26, 2006