[Physics] Why does the index of refraction change the direction of light


I've been studying in optics the macroscopic maxwell's equations, and how electromagnetic fields propagate through different mediums. Over there, the index of refraction appears, as a complex function that generally depends on $\omega$, with both refraction and absorption terms: $n_c=n+i\kappa$.

I understand how this affects the speed at which light propagates, both macro and microscopically, and how this affects to how light is absorbed when it transmits through the medium, but I don't get yet how this index changes the direction of light, producing dispersion. I mean, all time I'm seeing those effects as something that separates dispersive from non dispersive mediums and all that stuff, and I don't know how light is actually dispersed. As it's something that happens when changing medium, I guess it's an interface effect, and we haven't seen those effects yet, but I would like to know an explanation.

Best Answer

Here is an analogy that I like to use: (even though it is not really a correct physical explanation)

Imagine that you are out riding your segway over some strange surfaces, that each have a number $n_i$ that controls the speed that a segway wheel travels over it according to the formula $v_i=v_0/n_i$. Now imagine that you cross a straight boundary between two surfaces at an angle. Because of the angle, one wheel will cross the boundary before the other. If $n_i$ is higher for the entered surface this wheel will go slower than the other until it too crosses the boundary, which will cause the segway to turn towards the normal of the boundary. Similarly, if $n_i$ is lower for the entered surface, the first wheel to enter will go faster, and the segway will turn from the normal.

If you do the calculations for the segway you will get the the same results as for the wavefront explanation (basically Snell's law), but I really like how this analogy works with your intuition.

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