# How can electromagnetic waves be transverse?

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1. May 11, 2015

### daniel rl3

how can the electromagnetic waves be transverse and at the same time the E.M propagates in space (the definition of the transverse wave is that it is a wave in which it's medium particles propagate perpendicular to the direction of wave propagation) what i don't understand is that how can it propagate through medium while it is already at space

2. May 12, 2015

### Andrew Mason

Welcome to PF Daniel!

Where did you get your definition of a transverse wave?

For electromagnetic waves, the oscillation is not a material substance. Rather it is electric and magnetic fields that oscillate.

Propagation of light through a medium can be more complicated because of the interactions between the electric fields of atoms and the electric and magnetic field oscillations of the em wave. You should read https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/ [Broken] to get a better sense of how an electromagnetic wave propagates through matter.

AM

Last edited by a moderator: May 7, 2017
3. May 12, 2015

### daniel rl3

and about the definition, i saw it in a variety of websites and it was also written in my physics book.

Last edited by a moderator: May 7, 2017
4. May 12, 2015

### Andrew Mason

That definition of the transverse wave: "a wave in which it's medium particles propagate perpendicular to the direction of wave propagation" applies only to mechanical waves involving oscillations of matter. An electromagnetic wave is not such a wave.

A better, more general definition of a transverse wave might be: "a wave in which the oscillation of particles in the case of a mechanical wave, or of energy fields in the case of a non-mechanical wave such as an electromagnetic wave, propagate perpendicular to the direction of wave propagation". Generally, a transverse wave is the solution to the differential equation of the general form:

$\frac{d^2u}{dt^2} = c^2\frac{du^2}{dx^2}$

Maxwell recognized this kind relationship between time dependent electric and magnetic fields and saw that its solution was a transverse wave with speed $c = \frac{1}{\sqrt{\epsilon_0\mu_0}}$

AM

Last edited: May 12, 2015
5. May 12, 2015

### sophiecentaur

It seems likely, to me, that the statement related to transverse mechanical waves (e.g. waves along strings). There are no 'oscillating particles' involved when EM waves travel through space. However, when there are free electrons floating around (as in the Ionosphere) and the waves are at fairly low frequencies (HF and below), the individual electrons can be modelled as moving from side to side as the oscillating EM fields interact with them. But that is more advanced work and basic propagation through free space should be tackled first.

PS do not confuse this with the 'so called particles' of EM energy (photons) which do not wiggle about and have no defined position at all.

6. May 12, 2015

### insightful

Is it not accurate to say that for electromagnetic waves (or photons) that the electrical and magnetic fields oscillate at right angles to the direction of travel and are thus transverse waves?

7. May 12, 2015

### sophiecentaur

The waves are not photons. You cannot talk about both at the same time. The waves consist of varying transverse fields (as you wrote). That is nothing to do with the photons which are quantum particles and have no classical nature.

8. May 12, 2015

### insightful

So, there are no electric and magnetic fields in photons?

Last edited: May 12, 2015
9. May 12, 2015

### rumborak

Lol. No offense, that is such a convoluted statement, it adds to noone's understanding. More words =/= clarity.

I always thought the problem was the term "perpendicular", which suggests spatial extent. "Orthogonal" might be a better term to use, since it refers to general dimensions that are linearly independent and forces the reader to consider the matter on a more abstract matter.

10. May 13, 2015

### Andrew Mason

It was just the definition provided by the OP with an addition to include em waves. What is convoluted about that? Perhaps you could suggest a better one.

??

Perpendicular means 90 degrees or $\pi/2$ radians of spatial angle. In a transverse wave the motion of the matter or fields is in a direction that is 90 degrees to the direction of propagation of the wave.

AM

11. May 13, 2015

### Andrew Mason

The electric and magnetic fields are classical concepts Electromagnetic waves follow from Maxwell's equations. The photon does not follow from Maxwell's equations. While the photon interacts with electromagnetic fields it cannot be explained classically. In quantum electrodynamics the photon is the modeled as the carrier of electomagnetic force.

AM

12. May 13, 2015

### sophiecentaur

Perhaps 'associated with' would be a better way of putting it.
Perhaps you would like to try to 'draw' a photon, for yourself, and show how you imagine the E and H fields fit 'in' with it. You are trying to combine Classical and Quantum ideas together and you can hardly be blamed for that, bearing in mind how we are taught such things, initially. If you try to draw a photon, you have to give it a size and a position. How would you do that in a valid way? Feynman really didn't help when he drew his Feynman Diagram with a little wiggly thing, going from one particle to another. He knew what he meant and didn't, apparently, realise that not everyone is as smart as him and that we could all take the picture literally, rather than regard it as a symbolic, functional diagram. Treat it with caution.

13. May 13, 2015

### insightful

Ablosutely, EM fields "associated with" photons is much better. So, those EM fields associated with photons are transverse waves, right? I'm trying to get to an answer to OP's question. (Perhaps it's already been answered. OP?)

Last edited: May 13, 2015
14. May 13, 2015

### rumborak

I must apologize, I actually just realized something myself about this topic, which I had misunderstood before. I somehow thought that the term "perpendicular" was chosen lackadaisically, I.e. that they were really trying to say "well, it's not extending into space, so by definition it is perpendicular". I was essentially visualizing it wrongly as a scalar field. Only now did it click with me that indeed E and B's vectors are pointing perpendicular to the direction of travel.

15. May 13, 2015

### ZapperZ

Staff Emeritus
16. Jun 6, 2015

### daniel rl3

sorry i don't quite understand. didn't de brojle say that the photons have dual nature ( particle nature and wave nature) how come that the photons are not considered as waves? and if they act as a wave what kind of wave are they?

Last edited: Jun 6, 2015
17. Jun 7, 2015

### sophiecentaur

De Broglie said that matter can be described in terms of waves. Aamof it was not photons.
I tried to write what I meant concisely. The 'dual nature' does not mean they are the same. My message was that EM can be analysed from either standpoint, NOT that photons 'are' waves. Do not rely on my statement. Just read about this at a suitably informed level . (PF search could help, probably)

18. Jun 8, 2015

### Andrew Mason

It is very confusing. The photon is not modeled as a small electromagnetic wave. The λ or ν in Plank's law relates the photon energy to the wavelength of the light that the photon is associated with. The nature of the photon is something that puzzled Richard Feynman who helped to develop Quantum Field Theory to explain it.

AM