### Re: Are photons absorbed and re-emitted by glass?

Date: Mon Sep 17 11:27:14 2001
Posted By: Ronald Fisch, Physics, Washington University
Area of science: Physics
ID: 1000322720.Ph
Message:
```
There are many kinds of glass.  As I am sure you know,
the glass which is used in ordinary windows is almost colorless and
highly transparent.  By adding various kinds of atoms to the mixture
it is possible to make stained glass of any desired color.  And it
is also possible to make glass which is not transparent.

The index of refraction of a transparent glass is greater than one.
This means that the speed of light in this glass is less than the
speed of light in vacuum, since the speed of light in some medium
is equal to the speed of light in vacuum divided by the index of
refraction.  (This is essentially the definition of the index of
refraction.)  Actually, the speed of light in a material medium
is different for different colors of light.  This is called
dispersion, and it means that the index of refraction is a function
of the wavelength of the photons which make up the light.

Physicists sometimes like to say that the reason why light travels
more slowly in a transparent material is because the atoms are
absorbing and reemitting the photons, and this process slows down
the photons.  If the photons were absorbed without being reemitted,
then the material would not be transparent.  Mathematically, we can
describe absorption by an index of refraction which is a complex
number, having real and imaginary parts.

This idea of absorbing and reemitting photons is a description in
words of the equations which are used to calculate the index of
refraction.  Trying to explain equations with words often leads to
confusion.  To really understand what is going on, you must look
at the actual equations.

I hope that you find this explanation helpful.

```

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