For example, during refraction, when photons pass through a glass slab, they change direction. Why does this happen? Is the reason for this not applicable when photons strike perpendicular to the glass slab?
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$\begingroup$ You have to match fields to the boundary conditions. Coming in perpendicular to the slab means they boundary conditions are matched for the photon going straight on through. $\endgroup$– Jon CusterCommented Oct 30, 2023 at 16:25
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$\begingroup$ The direction is not changed by passing through the glass. The direction is changed at the interfaces between glass and air. $\endgroup$– Solomon SlowCommented Oct 30, 2023 at 17:17
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$\begingroup$ The equation that predicts the angle of refraction is 100% applicable to photons that strike perpendicular to interface. In that specific case, it predicts that the photon will be deflected by zero degrees. $\endgroup$– Solomon SlowCommented Oct 30, 2023 at 17:27
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$\begingroup$ It is actually not the density of the material that is important, but rather the speed of light inside that material. (Which is obviously closely related to density). Light refracts when it moves between media with different speeds of light. This could also be why light bends when moving past a star or galaxy, because of gravitational time dilation near those things light moves slower, and hence refracts, even though the space itself is of the same density. It is only the speed of the light that has changed. $\endgroup$– foolishmuseCommented Oct 30, 2023 at 17:49
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$\begingroup$ There seems to be great confusion also in the comments between photons, and electromagnetic radiation. Photons compose electromagnetic radiation but follow QED scattering dynamics individually, i,e, Feynman diagrams . To solve for a photon hitting a surface one has to model the Feynman diagram. $\endgroup$– anna vCommented Oct 31, 2023 at 5:52
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They change direction, or maintain the direction if the angle of incidence is normal to the surface, to minimize the time it takes to get from one point outside the slab to a point within the slab.
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$\begingroup$ That sounds like a reasonable answer to, "what is the shortest path from the point source A, which lies outside the glass body, to some point B that lies within?" But it's kind of unsatisfying if the question starts by asking about a particular photon. The photon does not know that it is bound for point B. It doesn't choose the best path to get there. $\endgroup$ Commented Oct 30, 2023 at 17:23
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$\begingroup$ Good points. But, nature actually works like this. It minimizes the action, which in this case it minimizes the time to get to the other point within the slab. You can compute this yourself. You have 2 media, the speed of the wave is c in the first material, and c/n in the second material (n >1). Now compute the time it takes for different paths, and you'll recover snell's law for the path that minimizes the time. $\endgroup$– JQKCommented Oct 30, 2023 at 17:51
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$\begingroup$ It's a bit complicated, and relies on quantum field theory, to explain why this works. $\endgroup$– JQKCommented Oct 30, 2023 at 17:51
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$\begingroup$ I repeat here :There seems to be great confusion also in the comments between photons, and electromagnetic radiation. Photons compose electromagnetic radiation but follow QED scattering dynamics individually, i,e, Feynman diagrams . To solve for a photon hitting a surface one has to model the Feynman diagram. $\endgroup$– anna vCommented Oct 31, 2023 at 5:53
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$\begingroup$ I don’t need Feynman diagrams to apply the principle of least time. $\endgroup$– JQKCommented Oct 31, 2023 at 13:30