Thursday, February 21, 2019

Why Does Light Slow Down In A Material?

Don Lincoln tackles one of those internet/online FAQs. This time, it is an explanation on why light slows down in water, or in matter in general.

Certainly, this is the explanation many of us know when we were in school. However, most of the questions that I get regarding this phenomenon came from people who want to know the explanation at the "quantum" level, i.e. if light is made up of photons, how does one explain this phenomenon in the photon picture? That is the origin of the two "wrong" explanations that he pointed out in the video, i.e. people wanting to use "photons" to explain what is going on here.

Actually, Don Lincoln could have gone a bit further with the explanation and included the fact that this explanation can account for why the speed of light (and index of refraction) inside a material is dependent on the frequency of the light entering the material.

Strangely enough, this actually reminded me of a puzzle that I had when I first encountered this explanation. If the electrons (or the electric dipoles) inside the material oscillate and create an additional EM wave, and the superposition of these two waves give rise to the final wave that appears to move slower in the material, then what stops this second EM wave from leaving the material? Is it only confined within the material? Do we detect "leakage" of this second or any additional wave due to things oscillating in the material? Because the second wave has a different wavelength, it will be refracted differently at the boundary, so it will no longer be aligned with the original wave after they leave the material, if they all leave the material.

Anyone knows?

Edit: Funny enough, and maybe because I watched this video, YouTube gave me an old MinutePhysics video that used the bouncing light particle explanation that Don Lincoln says isn't correct.


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