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Refraction of Light through Rectangular Glass Slab and Glass Prism

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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. Why is light bent back onto the same path when it leaves a pane of glass, but bent even further ‘off course’ when it leaves the prism?

2. Why is light at all wavelengths bent in the same direction when it leaves and 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.


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