I've never understand the idea that in non-polarized light, the "amplitude" of the light wave points in all directions, whereas in polarized light it points in only one. The amplitude of a light wave doesn't literally mean something is oscillating up and down, right?? The amplitude is just the light's intensity. So what does it really mean for light to be polarized? And why do thin slits cause polarization?
Light is an electromagnetic field: a wave-on-a-rope type thing except the rope is space. An electromagnetic field exerts a force on charged objects like electron, protons or even entire atoms and molecules. If you put an electron and shoot polarized light at it, it's movement will be consistent with the electric field going up and down in a particular plane. Analogously for unpolarized light.
A thin slit cause polarization the same way only one plane of oscillation is possible for a rope passing through a slit. Except what is stopping the oscillations in 'bad' directions is the interaction between the electromagnetic field and atoms that make up the boundaries of the slit.
Thanks! After reading this [0] I'm surprised that my high school recollection seems to be how it really works. It still seems very strange. One thing in that article bothers me though: it talks about the wave itself as vibrating, e.g. "A light wave that is vibrating in more than one plane is referred to as unpolarized light." But how does a wave vibrate? It seems like a sloppy use of language. Using your own words maybe it's more correct to say that space is vibrating? Or rather vibration is just a metaphor for the EM field?
The "length" of the photon is very short. There is not a long wave like you are thinking, like a wave on a string.
Take a short string and pull it over a sine wave. At any instant in time, one part of the string will be high, another part low. As you pull it, the position changes, but the sun total of all the "heights" in the sine wave is always the same, no matter what part of the sine wave you start at.
So you can think of a photon like that: As it snakes it's way through space, it doesn't actually move up and down, but rather as it progresses the front of it will sometimes be "high" and sometimes "low".
Take a hose and wave it in the air, to make a sine wave of water. Now think of each molecule of water - every single molecule only moves forward in a perfectly straight line! None of them move side of side. Yet it looks that way, but it's really new molecules, some of them are moving here and some there.
> in non-polarized light, the "amplitude" of the light wave points in all directions
The average is all directions. Each individual photon has just one direction. But photons are not like regular objects, if you send polarized light through a polarizer that is tilted 45 degrees relative to them you don't block the light entirely like you would expect, instead half the light gets through. I guess you could interpret this as each photon having a 50/50 chance of making it though.
> The amplitude of a light wave doesn't literally mean something is oscillating up and down, right??
Yes, it does mean that. The "something" is an electric and magnetic field. Does that count as a something?
Remember that a moving electric field induces magnetism, and a moving magnet induces an electric field. So the two fields essentially induce each other, that's why light can never stop moving, or even change speed - the fields would no longer induce anything.
> The amplitude is just the light's intensity.
No. The amplitude never changes. Intensity is the number of photons. A single photon doesn't really have an amplitude that way you would think.
I actually had the same misconception about light when studying it in highschool. I though that the up/down things that the teacher was drawing on the board was not literally how it worked (transverse wave) but that it was a measurement of the longitudinal wave. I think because I knew how sound worked, and I was thinking that way.
It turns out that it actually is a transverse wave in the electric field with another transverse wave in the magnetic field offset by 90 degrees. There is a good diagram of how it 'looks' here:
No, it's not a transverse wave in the sense that you're describing, like a wave in water. It's a wave in the E&M fields, both of which are 'vector fields'. Mathematically, we describe both electric and magnetic fields as a vector (a direction + length, like a little arrow) at each point in space. Light waves are oscillations in the lengths+directions of these vectors. There's no medium moving transversely, but the vector field points in the direction and with the magnitude drawn in those squiggly line diagrams.
See Maxwell's equations, which describe how those vector fields change over time. They do not describe how any sort of medium moves over time.
Yeah, you're right. I don't mean transverse in that it is a wave in water. There is no 'water' particles there to be moving up/down left/right.
Interestingly, Maxwell himself worked in a time when the aether model was dominant; that there had to be something for the waves to be moving in/through. His equations held up under both the aether model and special relativity which made aether unnecessary.
There's always going to be a limit to how well any macroscopic analogy can possibly describe what's going on with photons and light, but some metaphors work better than others. the trouble with the 'light is a particle and a wave' meme is that people try to combine two mental images - a cannonball flying through the air, and n ocean wave moving up and down, or a sound wave moving back and forth, and get a completely weird mental image of a cannonball flying along a sinewave path, or speeding up and slowing down. This isn't helpful.
Instead, you need to extend the metaphor in a different direction. Let's keep photons as cannonballs flying in straight lines. But now, imagine the cannonballs are constantly spinning. Some are spinning fast - they have a lot of rotational energy - they rotate with a high frequency. Others are spinning more slowly - low frequency, low energy cannonballs.
Now, which way are they spinning? Some have topspin or backspin, meaning they have a horizontal spin axis; some have sidespin - their axis of spin is vertical. Others might be spinning in a spiral - as if the barrel they came out of was rifled - so their axis of spin is aligned with the axis they're flying along.
You can imagine lots of cannonballs all flying along the same path, at the same speed, but they can all have different spin frequencies, and be spinning around different axes. Amplitude means more cannonballs. Higher frequency means more cannonballs spinning faster. Polarization means all the cannonballs are spinning round the same axis. Coherence (like in a laser) means all the cannonballs are spinning at the same speed and are pointing the same way as they pass the same point.
This maybe works better as a way to layer on the additional attributes a photon has - a frequency and a polarization - than to try to imagine some sort of transverse wave pattern. It also helps you deal with the idea of 'how do different photons carry different amounts of energy?'.
But, this is just a metaphor. Photons aren't really spinning cannonballs. It doesn't explain why thin slits cause polarization (and gives you a probably somewhat intuitive but definitely very wrong explanation for why bouncing photons off a surface causes polarization, so be careful.). It won't get you one jot closer to understanding quantum mechanics (no classical metaphor can do that), but it might just help you visualize how light can have mixtures of frequencies, polarizations, and amplitude, while still being just a bunch of particles.
What you're probably hearing is probability amplitude, which is the product of a wave function with its complex conjugate, rather than amplitude referring to wave intensity.
If you're interested in this stuff, check out the 160 or so videos from leonard susskind on youtube on modern physics.
