Saturday, May 04, 2019

Why Does Light Bend When It Enters Glass?

Don Lincoln tackles another "everyday" phenomenon. This time, he tries to give you an "explanation" on why light changes direction when it goes from one medium to another, and why some of the more popular explanation that have been given may be either incomplete, or wrong.

Certainly, any undergraduate physics student would have already dealt with the boundary conditions using Maxwell's equations, so this should be entirely new. However, he skipped rather quickly something that I thought was not handled thoroughly.

The continuity of the parallel component of E to the boundary is fine. However, Lincoln argued that the reason why the perpendicular component of the F field is shorter in glass is due to the polarization of the material, and thus, the sum of the light's E-field and the E-field from the polarization will cause the net, resultant E-field to be shorter.

But if the material's polarization can affect the perpendicular component, why doesn't it also affect the parallel component? After all, we assume that the material is isotropic. This, he left out, and at least to me, made it sound that the parallel component is not affected. If this is so, why?


1 comment:

Douglas Natelson said...

I agree that this particular point could have been presented more clearly. So, it's really the discontinuity in the polarization that actually matters, right? That is, what sets the difference in the surface-normal component of the electric field is the difference in (normally directed) polarization response between the two materials at the interface. That's not what happens for the in-plane part of the electric field. If I remember correctly without digging out my copy of vol 2 of the Feynman lectures, Feyman talks a bit about this, sort of a combination of the polarization picture and the Huygens picture, where the total electric field in a dielectric is a superposition of the propagating incident wave plus the radiation field generated by the oscillating charges of the medium (including an oscillating surface charge at the interface due to this discontinuity in the polarization).