Refraction and Speed of Light
How does light after having lost velocity while travelling
through a transparent medium regain its original velocity coming
outside into atmosphere. Which force is responsible for it?
There is no force involved, because there is no true change in energy involved.
For the purposes of your question, light does not act like a particle, it acts
like a wave. Waves will travel at different speeds in different media. An
example would be the difference in the speed of sound through water as opposed
to its speed through air. As light crosses from one medium into another, the
same WAVE will travel at different speeds.
To answer this question, you have to think about what happens to light
as it interacts with the medium. A light wave enters a transparent
material and excites an electron of an atom that makes up that
material. The excited electron now emits a light wave of its own,
which in turn excites another electron of another atom. This chain of
excite-emit-excite-emit continues on through the medium. Between
atoms, light is traveling at light speed (c = 3x10^8 m/s). However,
the time it takes to excite and emit contributes to the average time
it takes for a light wave to "travel" through the medium. So you see,
light never really slows down -- it just appears to because of the
interactions that take place at the atomic scale. In glass, the
apparent speed is about 2 x 10^8 m/s. In diamond, about 1.2 x 10^8 m/
s. Hope this helps.
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Update: June 2012