Follow up to this post to illustrate the contradictions.
Light going through a pane is easy enough. All colours are bent by the same amount on entering and on leaving, and what goes in = what goes out, which is parallel to the original ray. We also know that light takes the fastest route from source to image (like the "dog crossing a canal diagonally to chase prey some way from the canal bank" problem in maths):
Light going through a prism is easy enough as well. For some reason, longer wavelengths (red) are bent less than shorter ones (blue). You can read this one in either direction - either white light source on left being split into colours, or different colours coming in from the right to merge to white light on the way out (left):
If you add a second prism flipped upside down with a gap between them, the second prism reverses everything back into a single white beam on the way out - the two prisms act much like a single pane with an air gap in it.
But what if the two prisms are polished so perfectly that they are flat on a molecular level, and you move the two prisms closer and closer together until they are 100% touching (forming a single pane). We know what goes in, we know what comes out.
- Do the different colours take different paths (as shown in my rather badly drawn picture below), or do they "realise" that they are being tricked and "decide" that they are actually dealing with a single pane and then all follow the same path (as in the first picture)?
- They can't "know" what to do on entering until they leave again. Or does the fact that light, in its own reference frame, travels instantaneously (if something is travelling at the speed of light, then time slows to zero), play a part here?
- The assumption that they all take the fastest route (in our frame of reference) from source to image breaks down, as the red light (being least bent) must have travelled a shorter distance through the glass than green and blue.
- Or, as I asked last time, is there some mysterious quantum mechanics effect happening on the tiniest level incomprehensible to humans, which means that each photon takes every possible path, has a "think" when it's got there and then "chooses" the best one?
Monday, 11 July 2022
Light passing through a pane, a prism and two prisms
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8 comments:
"..is there some mysterious quantum mechanics effect happening on the tiniest level incomprehensible to humans, which means that each photon takes every possible path, .."
Well, sort of. That's the basis of Feynman's "sum over histories" or path integral approach. Not easily explained, let alone understood (at least by me).
His short book "QED the strange theory of light and matter" and/or the video of one of his lectures
http://vega.org.uk/video/programme/46
might help. Or confuse further.
Vfts, thanks, I won't understand it, but at least I know it's a bit weird.
The reason why the light exits the second prism as white light is simply because whatever is happening when the white light hits the first prism is a fully reversible mechanism. It is what we would expect. The puzzle is why the light is split in the first place. That's where the likes of Feynman come in. In my previous comment on the matter, I wasn't addressing this bit, so I wasn't glossing over anything.
Hi Mark, I gués I misunderstood your question last time. I dont know the answer either. Feynman's little book on QED is worth a read.
MiM, you pointed out my mistake on my Q1, which helped. The joy of all this is that the typical diagrams contradict each other, that was my point. A bit like all the contradictions in MMGW theory.
"Light going through a pane is easy enough. All colours are bent by the same amount on entering and on leaving..."
No. Each colour is bent by a different amount on entering and back by the same different amount when leaving. E.g. red might be diffracted by 20 degrees, blue might be diffracted by 21 degrees . On exiting the pane, red would be diffracted 20 degrees, blue by 21 degrees so because the light beam is wide and the pane of glass is thin, the diffraction is barely noticeable. The narrower the beam and the thicker the pane of glass the more evident it would be.
Your third diagram is wrong too, the different colours would not be diffracted at the plane where the two prisms meet. In fact, if they were separated it would make it more obvious. The colours would be diffracted exiting the first prism,as in your second diagram, then diffracted back parallel with their original line through the first prism.
Does that make sense?
Sorry, I think I meant refraction, not diffraction. In my defence, it's almost 60 years since I did this stuff for GCE 'O' level.
F, ta. I get the first bit, so with really thick glass, what you see is tinged red at one edge and tinged blue at the other?
I knew my third diagram was wrong. Clearly, nothing would happen at the interface between two prisms. I was trying to highlight the contradiction between the traditional "pane" and "prism" diagrams. But ta anyway, makes sense.
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