# Light as a wave ? Of what?

1. Jun 18, 2009

### hokusai

Light as a "wave"? Of what?

If I watch waves travelling through water (say, a pond) I can see that the actual water molecules are moving, transversely to the direction of the wave; To put it simply, a given water molecule will be closer to the bottom of the pond at a wave-trough than at a wave peak. The distance between peaks (or troughs) gives the 'frequency' of the wave, and the distance between a molecule's position at a peak and at a trough gives its amplitude.

Am I right so far?

Similarly, 'if I 'observe' a sound wave (in air) , I shall find that air molecules are more closely compressed, hence bunched together, at some times (the 'peaks') than at the troughs; the sound wave is thus a compression, or fore-and-aft, wave. Yes?

The 'direction of motion' of such a wave -- the way it spreads across a pond, for instance -- doesn't in fact imply the physical movement of anything in the direction of the wave. But, none the less, something is moving, transversely to, or fore-and-aft with respect to, the wave, and the movement of those 'somethings' -- water molecules, air molecules, or whatever -- can be observed with respect to their movement.

My question is this :- What is it that is moving in the case of a light wave? The wave, too, has a frequency (which, over some range, determines its 'colour') and an amplitude, which determines its brightness. And, since light can be polarised, we can determine 'the transverse direction of its amplitude' (if you see what I mean).

But WHAT is it that is moving transversely to its direction of propagation?

And, if the answer is "Nothing" -- then what do we mean by saying that light has -- or, at least, can sometimes have -- the attributes of a wave ?

Martin Woodhouse [ http://www.martin-woodhouse.co.uk ]

2. Jun 18, 2009

### Hootenanny

Staff Emeritus
Re: Light as a "wave"? Of what?

Nothing is 'moving' perpendicular to the direction of propagation, rather the electric and magnetic fields are oscillating perpendicular to the direction of propagation (as well as perpendicular to themselves). See http://en.wikipedia.org/wiki/Light#Electromagnetic_theory" for more information. Apologies that it is only a short Wiki piece, but I don't have a better reference to hand.

Last edited by a moderator: Apr 24, 2017
3. Jun 18, 2009

### HallsofIvy

Staff Emeritus
Re: Light as a "wave"? Of what?

Light is a wave in the electro-magnetic field, the "electric and magnetic fields" Hootenanny refers to.

4. Jun 18, 2009

### fleem

Re: Light as a "wave"? Of what?

When light goes through a medium containing electric and/or magnetic dipoles or particles, we see the dipoles swing to and fro similar to the way water moves when a wave goes through it. Those dipoles are certainly contributing the lion's share to the behavior of light through that medium. Space, itself, also has the attributes of such a medium (permittivity and permeability)--except that permittivity and permeability of space doesn't drop off with the cube of the distance from any dipoles, and also it has (as far as we know) flat frequency response--in fact we aren't too sure what dipoles are even contributing to the permittivity and permeability of space! So we don't really know what electric fields in space are. We only know how to measure and predict (mostly) how they will act. There are a lot of things about light that are like classical waves, but light is not a classical wave. There are also a lot of things about light that are like classical particles, but light is not a classical particle. The real reason is that there is no such thing as classical--except as a tool to estimate macroscopic behavior. But even so light is still not as classical as sound or water waves. Conjectures of what light really is are a subject for other threads, i think.

5. Jun 18, 2009

### hokusai

Re: Light as a "wave"? Of what?

Thanks to all, esp. fleem. It's just that now, at 77 years of age, I find myself asking the same questions of physics -- which, effectively means, of science -- as I asked in my teenage mind. and finding as few real answers. I share, for instance, your apparent puzzlement (or at least ambiguity) about space, fleem. I can see there's a difference between the sort of vacuum we can create in the laboratory and the real, no-fooling vacuum of 'empty space': but if there is such a 'thing' [ ! ] as empty space then we also have action-at-a-distance; but, worse:-- if there is 'nothing' (that is, empty space) between X and Y then X and Y are touching one another . . . yes?

And we haven't even got on to quantum physics, Heisenberg, Copenhagen, old Shrodinger's pussy-cat, and so forth. Not to mention strings . . .

6. Jun 18, 2009

### Andy Resnick

Re: Light as a "wave"? Of what?

Not quite- those waves you describe are properties of the water-air interface, not of the bulk phase of water. The sound waves you describe later are longitudinal waves, but the surface waves you mention here are different, and rely on different physical phenomena to explain the propogation speed, dispersion relation, etc.

What propagates is a disturbance in the electromagnetic field, much as a surface wave is a propogation of a disturbed surface. We can say, in the case of fluid surface waves, that the interfacial energy is providing the restoring force (the frequency), while in electromagnetic radiation the physics is different- the interplay between electric and magnetic fields provides the 'motive force'.

We say that light has some wave attributes because under some circumstances (diffraction, for example), the physical behavior of light can accurately be describes as waves.

7. Jun 18, 2009

### Staff: Mentor

Re: Light as a "wave"? Of what?

This, at least, is easy. Light exhibits all of the following wave behaviors:
diffraction
refraction
interference
reflection
Doppler

In addition light is classified as a transverse wave because it exhibits polarization which is a property of transverse waves rather than longitudinal waves.

None of this relies on any description of the mechanism of light or its medium, it is all simply observed behaviors. It does not matter what is "waving" or how it is waving, light is a wave simply by virtue of the fact that it behaves as a wave.

8. Jun 19, 2009

### granpa

Re: Light as a "wave"? Of what?

I dont understand that at all.

9. Jun 19, 2009

### Born2bwire

Re: Light as a "wave"? Of what?

I didn't want to be the one to respond to that, but yeah. Not every medium has dipoles that will respond to an incident field and dipoles are most certainly not necessary to propagate the wave. The wave has no medium of propagation, but a medium will affect the propagation of the wave should one (or more) be introduced. These microscopic effects are generally macroscopically modelled (classically) by the polarization of dielectrics by electric fields and the magnetization of magnetic dipoles. However, most mediums in life do not experience magnetization outside of the common ferromagnetic materials like iron. We have a pretty damn good idea about what light is. Classically, light is an electromagnetic wave consisting of self-supporting electric and magnetic fields. Quantum electrodynamically, light is propagated by photons.