|MadSci Network: Physics|
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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