In what direction does a light wave vibrate?

[Jordan Regan]

I've wondered this for a while but not known how to ask the question,

If light is a transverse wave, then what is it transverse to?

To elaborate, light travels in three-dimensions, radially. To me, this seems analogous to the sound wave, with pulses of pressure moving longitudinally to the direction of travel. The only problem is that I've had it drilled into me that light is a 'transverse' wave, and imagining this in three dimensions seems nigh-on impossible.

I'm guessing that it has something to do with the complex plane, the magnetic coupling that comes with the wave and even probably on a very deep level, a multi-dimensional understanding of space.

Thanks

 

[A.T.]

If light is a transverse wave, then what is it transverse to?

https://en.wikipedia.org/wiki/Transverse_wave (the very first sentence)

 

[Jordan Regan]

https://en.wikipedia.org/wiki/Transverse_wave (the very first sentence)

Thanks for helping me to find the Wikipedia page for a transverse wave - I can now comfortably say that according to one un-cited sentence (the very first sentence), my definition of 'transverse' has been confirmed.Anyway, to clarify (as my query remains to be concluded), I can imagine particles radiating from a source with some electromagnetic wave-function attached to them that has an oscillation transverse to the direction of motion. This bit is simple to understand, provided light can be quantized.

The problem comes when I try to understand the actual act of a wave propagating in three dimensions: I know that a light source will transmit information in all directions in 3-D space, and that through the famous diffraction experiments, wave-functions in the electromagnetic field are imposed together.

Imagining an array of photons, sharing one superimposed wave-function, what would the wave look like in terms of its transverse nature?
Would the waves superimpose to form an overall longitudinal wave, for example?

This is complicated when considering polarity, I know, but now you should hopefully be able to see where the holes in my knowledge and understanding are.

Again, thank you

 

[Ibix]

Light waves don't really look like anything. As a light wave passes through a point, the electric and magnetic fields at that point gain and lose strength, and the directions associated with them are always perpendicular to the direction of travel of the wave (hence its transverse nature).

It's important to realize that nothing actually moves in an electromagnetic wave, unlike in a water wave. They're often depicted as a series of rising and falling arrows perpendicular to a light ray, but this is slightly misleading. It's an attempt to visualise the changing field at a point as a series of arrows of different lengths, but in reality there aren't any arrows. There's just changing field vectors.

 

[A.T.]

This is complicated when considering polarity,

It's actually much simpler to visualize for polarized monochromatic light.



For non-polarized polychromatic light the E/H field vectors are still constrained to a plane, perpendicular to propagation. But their direction and amplitude within that plane don't change in that regular fashion.

 

[tech99]

t I've had it drilled into me that light is a 'transverse' wave, and imagining this in three dimensions seems nigh-on impossible.

As with many conceptual ideas in optics, I prefer to look at a low frequency version, so look at the radiation of radio waves from a dipole. If the conductor is vertical, the radiating electrons are accelerating up and down the wire and this defines the direction of the radiated electric field and the polarisation. If you are not on the equatorial plane, the geometrical situation can be visualised. There is no radiation off the end of the dipole.

 

[Jordan Regan]

This discussion is clearing things up for me, mainly,

- I forgot that accelerating particles cause electromagnetic radiation, this both implies that there's always a defined polarity, and that wave-functions strictly accompany particles.
- The field vectors are representative of changes to the field strength, and in reality, a transverse wave doesn't have to be easy to visualize (really the arrows can be as large or small as we want, and are defined by the units of potential).

I think I can comfortably move on with my work now; as it happens, I'm studying Planck's constant for my lab report on the photoelectric effect, and want to relate the wave-function to the particle nature of photons with a deeper understanding.

The comments have been largely helpful, thank you all!

J.R.

 

[davenn]

- I forgot that accelerating particles cause electromagnetic radiation,

and you forgot ...

accelerating charged particles cause electromagnetic radiation

 

[Jordan Regan]

So what happens in a decay then? When light is generated either by an annihilation of opposing particles or otherwise through a radioactive decay, what determines the polarity of light in this case?

 

[vanhees71]

Then the polarization is random, according to the rules of quantum theory. Search the forum for pion decay to two photons, $\pi^0 \rightarrow \gamma+\gamma$. We've recently discussed the issue at length for this example.