Probably a candidate for one of those “things I should have known that I learned embarrassingly late in life,” but…
I’m not talking about wave/particle duality or anything complicated like that. My question is much more basic. Picture in your head the standard “graph” of a wave: a sine wave centered on the X axis. This graph is drawn a hundred times a day around the world, I’m sure, with little peak-to-peak labels for “wavelength” and little peak to valley labels for “amplitude,” and probably a reciprocal somewhere nearby for “frequency.”
I was thinking about this the other day for light, and realized that I was picturing little photons tracing out a sine wave in space, and thought “well, that’s pretty stupid.” The x-axis is obviously time, but what is the y-axis really measuring? What is it that’s actually forming a sine wave, and in what medium? Is it some sort of quantum probability cloud of a photon’s presence in space? If I snap a shutter closed halfway through the tracing of one of those peak-to-peak wavelengths, what is it that I’ve got half of on each side? For sound, it’s pressure: but sound and pressure aren’t quantized units like photons. What is it for light (or electromagnetic energy in general?)
The light wave is, in fact, the wave function of the photon, but of course that wasn’t the original way it was described. Usually, it’s described in terms of the strength of the electric or magnetic field. A light wave is composed of both (hence, “electromagnetic wave”), at right angles to each other. So if you have a light wave propagating in the X direction, you’ll have an electric field in the Y direction, and a magnetic field in the Z direction (or vice-versa, or some diagonal combination, but they’re always at right angles).
Or position. It being a wave, you can’t tell the difference between looking at the same point in space at different times, or looking at many points in space all at the same time.
Its the strength and direction of the electric field. (or the magnetic field, which looks the same, but is 90 degrees out of phase and points at right angles to the electric.)
Intensity is amplitude squared. For visible light, it’s very difficult to measure or experience the amplitude directly, but it gets easier at longer wavelengths, and once you get down to radio waves, it’s always the amplitudes you’re working with. The changeover point where you can easily deal with either depends on precisely what equipment you’re dealing with, but is generally somewhere in the vicinity of microwaves.
EDIT: Man, people keep on posting in between me.
Actually, unless you’re dealing with light propagating through some funky material, the magnetic field is in phase with the electric.