Here is a great explanation of how to work out what comes out of the other side of the slab/prism. I'm sure he'd get top marks in an exam, and on the basis of 'what goes in and what comes out' we know the result is correct.
But what nobody can explain - a proper physics prof. explains the head scratching in this video - are the striking differences, which appear to be down to weird quantum stuff and the wave/particle duality. And he's just talking about light travelling more slowly through a pane, he doesn't mention prisms (I don't think).
The different behaviour through a pane vs through a prism struck me a couple of days ago. The unanswered questions are:
1. UPDATE. Following Mark In Mayenne's comment, I rethought this one, there is no contradication here.
2. Why is light at all wavelengths bent in the same direction when it enters and leaves the pane (so barring reflections and imperfections in the glass, the image is the same on the other side), but is split by frequency when it enters and leaves the prism (so the image is quite different on the other side)?
3. What would an observer see if he were inside the pane/prism, looking towards the light source? At that stage, the light hasn't been bent again on leaving and the light doesn't 'know' if it's in a pane or in a prism? Would the light be split by frequency or not?
4. With a pane of glass, you can assume that the light behaves as a wave. When waves in water reach shallower or deeper water, they change direction in predictable fashion (regardless of frequency - all light travels at the same speed). Does light behave as particles in a prism? Does that go some way towards explaining the differences?
5. It's a like the 'dog running from A to B via swimming a canal diagonally' problem in maths. The actual calculation is heinously complicated (simplified version here, and even that one stumped a lot of people), but somehow dogs seem to be able to do this calculation by instinct and choose the fastest route.
With a pane of glass, the light (which travels more slowly through glass) takes the route from source to end point along the route that takes the shortest possible time, almost as if it 'knows', or as if takes every single possible route and then 'chooses' the fastest. So does the light somehow already 'know' whether it's entered a pane or a prism as soon as it enters?
The joy of all this is that nobody seems to know and probably never will, least of all me. I guess that's quantum mechanics for you. Light is inanimate, it surely can't 'know' anything, can it..?
Something to think about when you're at a boring event or you can't get to sleep.
Thursday, 7 July 2022
"Refraction of Light through Rectangular Glass Slab and Glass Prism"
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11 comments:
And then there is Islandic Spar. Or calcite.
Weird.
Dh, l had to look that up. Aargh, that's even more mental.
1) The sides of a pane of glass are parallel. In a prism the entry and exit sides are at an acute angle so the light is diverted even more.
MiM, damn, I'll have to think about that a bit more...
Point 3. AFAICR, all the physics diagrams showing light passing through a prism show the white light being split by frequency on entering the prism, then further split by frequency on exiting it. All that happens with a pane of glass is that the light entering at an angle is split by frequency whilst travelling through the glass, then the reverse happens on the other side. No doubt this could be demonstrated using two prisms oriented so that their outer surfaces were parallel. Light would enter the first prism as white, exit it as a rainbow, enter the second prism as a rainbow and exit it as white light.
Spot on, Bayard. And I believe that Newton actually did that experiment and got exactly that result.
"Nature and Nature's laws lay hid in night:
God said, Let Newton be! and all was light".
MiM, item 1 now resolved, thanks.
B and D, sure, we know 'what goes in' and 'what comes out' but that does not resolve 'what happens inside'.
With a pane, all frequencies follow the same path and come out at the same point in sync.
With two prisms, they follow different paths that magically meet up at the same point. Some frequencies will have travelled a longer path than others, so can't be in sync.
TomJ, I am sure that other countries which were at the forefront on glass technology in the 16th century (Italians, Dutch?) have rhymes celebrating their own folk heroes.
3a) What is the difference between a solid pane and two perfectly polished prisms that effectively form a plane?
3b) What does the observer INSIDE the pane see vs what does the observer inside two prisms see ?
" we know 'what goes in' and 'what comes out' but that does not resolve 'what happens inside'."
Yes it does, because, with the two prisms experiment, the area between the prisms is the "inside".
Another way to find out 'what happens inside' is to make a very thick pane of glass out of a slightly cloudy glass. Then, if you look in at the edge of the pane, you should see the coloured light being slightly scattered by the cloudiness on its way through the glass, before recombining as it exits the pane on the other side.
"With a pane, all frequencies follow the same path and come out at the same point in sync."
No they don't, they all forllow parallel paths that are of the same length. Imagine a file of soldiers walking up one side of the road and wanting to cross to the other sides. They could all follow the same path, by each soldier crossing the road when he got to a certain point. Alternative, to reduce the crossing time to a minimum, they could all start to cross the road simultaneously, then turn and start walking up the other side when they simultaneously reached it. The latter, not the former, is how light passes through a pane of glass.
B, I have done a follow up post to show what you are glossing over and the questions that remain unanswered.
"It did not last: the Devil howling, 'Ho!
Let Einstein be!' restored the status quo".
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