# What forces do act upon light?

1. Feb 10, 2012

### lightconstant

I am not able to understand light although I guess I am not the only one since It is such an abstract subject.
When light with its own duality wave-particle goes through the air, does it experiment any collision against it?
Because air must have particles, atoms... should not there be a collision, a elastic or inelastic one? Or this does not happen because It is a wave?
Imagine a strong wind It is travelling at huge speed, I believe this wind does change the direction of sound, I do not know about the speed, What about light will it be affected by the strong wind if not why not?
Wind is an example but it could be any other force if I push an object this object moves if I could throw my arm at huge speed into light will this affect its direction, intuitively the answer looks no but I want to know the reason.
Another way to ask this question in a more elegant way would be:
What forces do act upon light?
I heard gravity do acts on it, bending it, black holes... maybe some electromagnetic forces as light It is an electromagnetic wave could but I think I read this is not the case, do we know any other force outside gravity that affect light?

2. Feb 10, 2012

### ghwellsjr

Yes, air does effect light. That's why the sky is blue as opposed to transparent. That's why the apparent speed of light is slower through air. The photons hit molecules of the gas that makes up air and new photons are remitted, sometimes at a different wavelength, maybe a different direction, heating up the air and changing their energy. That's why Einstein said "in empty space" in his second postulate.

3. Feb 10, 2012

### lightconstant

Thanks ghwell maybe I have explained bad myself, I know light travels at different speeds in different mediumd due to the refractive index but what I wanted to know is if light is affected without changing the refractive index?
Let's see if I explain it correctly suppose that light is travelling at some speed in the air now a strong wind comes and hits the light, there are two possibilites either :
-light changes its speed.
-It does not.
Which one does it happen and if light does change speed travelling through the air receiving the wind impact is this because the refractive index has changed as well?
Is the refractive index different with air and with air plus wind?
Is light constant in air?
We can say It is constant because It is not affected by wind or We can say it is constant if It is affected by wind because now we are in a new refractive index?
Does the refractive index depends on the speed of the medium?

4. Feb 10, 2012

### ghwellsjr

The photons all travel at c. That's why I said the apparent speed is slower through air. In this case we're talking about the average of a huge number of photons. But the individual photons can go a long way before they hit a molecule of air. If the air is denser or contains more moisture or even large molecules of water (think clouds) then the photons have no chance of getting through until they hit a molecule. The speed of wind is insignificant compared to the speed of the photons but winds are caused by pressure differences which are caused by temperature differences so any given wind could be the result of a lower density of air or a higher density of air. In the one case more photons get through and in the other case fewer get through but the difference in average speed is very small.

These questions are really not related to relativity and should be more properly dealt with in one of the other forums.

5. Feb 10, 2012

### lugita15

This is slightly misleading; see this link. The photons do not actually get absorbed by atoms in the medium. Instead, what happens is that some of the energy of the electric field associated with the photon gets temporarily absorbed into the vibrational energy of the lattice, and then the lattice vibrations give back the energy to the electric field of the photon. In the process of all of this the photon is not harmed in any way. But the flow of energy has been impeded because that it spent some time in the vibrational modes (phonons) of the lattice. So the progress of the electromagnetic wave group and thus the progress of the photon has been impeded. In a very real sense, the speed of the photon has slowed down.

In any case it's not about photons traveling at c but getting absorbed and re-emitted by atoms. That process does occur in nature, but it is not what is responsible for the speed of light in a medium.

6. Feb 11, 2012

### pervect

Staff Emeritus
The density gradient of the atmosphere causes light to bend, enough so that surveryors have to compensate for it.

See for instance http://www.aboutcivil.com/curvature-and-refraction.html [Broken], http://mintaka.sdsu.edu/GF/explain/atmos_refr/bending.html

Light usually bends downward - but only at about 14% of the rate that the Earth curves. Atomspheric temprature profiles can affect the amount and direction of this bending.

It's not particular convenient to describe the path of light using forces - if it's possible at all. Trying to imagine a density gradient causing a force is probably not going to get you very far.

It's much easier to apply Fermat's principle - the idea that light minimizes the optical path length.

See for instance http://en.wikipedia.org/w/index.php?title=Fermat's_principle&oldid=466421234

Fermat's principle is a specif example of physics based on Hamilton's principle, a very powerful, general, and useful replacement for the "forces" taught in freshman physics that is generally not introduced until graduate school.

Last edited by a moderator: May 5, 2017
7. Feb 11, 2012

### willie001

It's much easier to apply Fermat's principle - the idea that light minimizes the optical path length.http://www.amzcard.info/g.gif [Broken]

Last edited by a moderator: May 5, 2017
8. Feb 11, 2012

### ghwellsjr

9. Feb 11, 2012

### Q-reeus

Actually lugita15's correction in #5 itself needs some correction. There is no 'lattice' in a gas such as air, and thus no collective photon/phonon mode interactions a la Zappa's article which specifically deals with solids.
That article is not all that accurate itself. First sentence of the very long 2nd paragraph there: "The better way is to imagine light as a wave, which it is, that compresses and stretches the electric and magnetic fields of space." Wrong - there is no 'compressing' and 'stretching' of (implied pre-existing) E and B fields. They are generated ab initio by the source distribution and at large distances propagate as mutually orthogonal radiation fields through an assumed otherwise 'empty' vacuum/medium - that's all. (I don't want to get into a discussion here about whether a classical EM radiation field should or shouldn't be treated as a collection of photons 'en route' - there is debate about that amongst QED pros) The rest of that long-winded article obscures the simple fact that in typical gaseous media at typically optical or below frequencies, gas molecules respond as weak dipoles that individually do indeed randomly re-radiate. But owing to the vast numbers - *and progressive phase differences*, the collective re-radiated wave adds to the initial wave such as to merely slow it overall a tiny amount. A brief sumary of propagation through gaseous media: http://www.miniphysics.com/2010/11/propagation-of-light-through-gaseous.html [Broken]

Last edited by a moderator: May 5, 2017
10. Feb 11, 2012

### Naty1

Q-reeus: good post....I agree.
I was also going to comment that something seems incorrect in all three explanations.....

Would someone like to improve post #5, which is pretty darn good, so we have a concise, agreed upon explanation?

Last edited: Feb 11, 2012
11. Feb 11, 2012

### Naty1

I happened to be here just a few minutes ago.....LOTS going on when light passes through a solid...maybe too much for a reasonably accurate one paragraph description:

http://en.wikipedia.org/wiki/Translucent#Light_scattering_in_solids

12. Feb 11, 2012

### Q-reeus

Thanks naty1. Looking back at the OP's questions, seems he is mainly interested in forces - or rather fields, that effect light. While it is normally taken that in vacuo a magnetic field has no effect, there is a tiny influence at very high field strength - see 2nd answer given here: http://answers.yahoo.com/question/index?qid=20100419182049AA1Ww4u

13. Feb 11, 2012

### lugita15

I was also talking only about solids in my post.

14. Feb 11, 2012

### Q-reeus

Fine I won't dispute that's what you were actually thinking about, but your quote of ghwellsjr in #5 was referring to propagation through air.

15. Feb 11, 2012

### Antiphon

Actually, the photons are absorbed and re-emitted by the phonons.

What you describe is inconsistent. An electric field can't give energy to the electric field of a photon. The entire electric field is made up of photons and they in turn can only act on charges in the material.

The detail of how a wavefront passes through a dielectric is that there is a leader signal that makes it through the material at c. There is significant absorption while the material is being "vibrated up" by the incident wave. Once the whole slab is vibrating, it becomes a coherent radiator launching a new wave in the direction of the old one with a group delay.

16. Feb 11, 2012

### lugita15

Look at this post by ZapperZ; it makes the same point as the FAQ entry and my post.

17. Feb 11, 2012

### Antiphon

Yes; ZapperZ's point is that the absorption and re-emission is not by the atomic spectra but by the phonons formed by the larger material matrix. In non quantum lingo this is called the dielectric polarization.

18. Feb 11, 2012

### lugita15

My understanding is the following: If the frequency of the light is compatible with the normal modes of the lattice, then you can have genuine absorption of the photon, and then the photon can be re-emitted by the phonons. But that is not the effect that is primarily responsible for light slowing down in a medium. Rather, what usually happens is that the photon is not oscillating at a natural frequency of the lattice, so that some of the energy of the photon is taken away by the lattice, and then the E-field of the photon gets it back after a delay. The net effect of this is that the energy flow, and thus wave group, is lagging behind where it would have been if there had been no lattice. Thus the speed of the photon has been effectively reduced.

19. Feb 11, 2012

### Antiphon

How do you take some energy from a photon?

20. Feb 16, 2012

### ApplePion

This is an interesting question. Here is how you would solve it.

Suppose you have light travelling in air that is moving. We will call the speed of the air "u" Make a Lorentz transformation so that in the new frame the air is not moving. In this frame the speed of the light is a little less than c. We will call it "v". Now transform back to the frame where the air is moving at a speed of u. The speed of the light in this frame can be found by the velocity addition formula, and it is (u + v) /(1 + uv/(c^2)) This is not the same as v, and thus the speed of light is different in a moving medium than it would have been had the medium not been moving.