Refraction and Acceleration
Name: Christopher S.
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
Dr. Ken Mellendorf
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.
Volume I, Chapter 31-1 through 31-6, which describes refraction, dispersion,
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.
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
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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Update: June 2012