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Name: Christopher S.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2001-2002

Why is it that when light travels from a more dense to a less dense medium, its speed is higher? I've read answers to this question in your archives but, sadly, still don't get it. One answer (Jasjeet S Bagla) says that we must not ask the question because light is massless, hence questions of acceleration don't make sense. It does, however, seem to be OK to talk about different speeds of light. If you start at one speed and end at a higher one, why is one not allowed to talk about acceleration? Bagla goes on to say that it depends on how the em fields behave in a given medium. It begs the question: what is it about, say, Perspex and air that makes light accelerate, oops, travel at different speeds? If you're dealing with the same ray of light, one is forced to speak of acceleration, no? What other explanation is there for final velocity>initial velocity? Arthur Smith mentioned a very small "evanescent" component that travels ahead at c. Where can I learn more about this? Sorry for the long question. I understand that F=ma and if there is no m, you cannot talk about a, but, again, you have one velocity higher than another for the same thing. I need to know more than "that's just the way em fields are!"


An explanation that satisfies me relates to travel through an interactive medium. When light interacts with an atom, the photon of light is absorbed and then emitted. For a moment, the energy of the light is within the atom. This causes a slight delay. Light travels at the standard speed of light until interacting with another atom. It is absorbed and emitted, causing another slight delay. The average effect is taking more time to travel a meter through glass than through air. This works like a slower speed. An individual photon does not actually slow down. It gets delayed repeatedly by the atoms of the medium. A more dense medium has more atoms per meter to cause delays.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Congratulations! on not being willing to accept "that is just the way em fields are!" The answer to your inquiry is not all that simple (my opinion), but I won't try to do so in the limited space allowed here, not to say my own limitations of knowledge.

Like so many "simple" physics questions, I find the most lucid, but accurate, explanation in Richard Feynman's, "Lectures on Physics" which most libraries will have. Specifically, see Volume I, Chapter 31-1 through 31-6, which describes refraction, dispersion, absorption, and diffraction. The "answer" has to do with how matter alters the electric field of incident radiation, but I won't pretend to be able to do a better job than Feynman.

Good question,

Vince Calder

The answer is that you are not dealing with the same ray of light. In vacuum a photon just keeps going at the speed of light. In a medium, however, it interacts with the atoms, often being absorbed while bumping an atomic or molecular motion into a higher energy state. The excited atom/molecule then can jump to a lower energy state, emitting a photon while doing so. This can obviously make light appear to travel slower in a dense medium.

In detail, it is a very complicated question, requiring at least a graduate course in electromagnetism to begin to understand. Why, for example do the emitted photons tend to travel in the same direction?

Best, Richard J. Plano

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