Here is the information you want, from Wiki:
When light enters materials, its energy is absorbed. In the case of
<http://en.wikipedia.org/wiki/Transparent_material>, this energy is
quickly re-radiated. However, this absorption and re-radiation
introduces a delay. As light propagates through dielectric material it
undergoes continuous absorption and re-radiation. Therefore the speed of
light in a medium is said to be less than /c/, which should be read as
the speed of energy propagation at the macroscopic level. At an atomic
level, electromagnetic waves always travel at /c/ in the empty space
between atoms. Two factors influence this slowing: stronger absorption
leading to shorter path length between each re-radiation cycle, and
longer delays. The slowing is therefore the result of these two
The refractive index of a transparent material is defined as the ratio
of /c/ to the speed of light /v/ in the material. Larger indexes of
refraction indicate smaller speeds.
I am going to try to explain in writing what I am concerned about with
your behaviour. It is up to you whether or not you ignore it, but you
must remember that you, like everyone else, is dependent on others for
gaining knowledge and for social interactions and if they find it
unpleasant to interact with you, it is you, not they, who will suffer.
You react defensively to most suggestions, mild criticisms or rebukes.
Sometimes your reaction is justified, but a lot of the time it is not
and you have never learned to just agree with someone pleasantly, take
on board what they are saying and save the fight for when it really matters.
You are becoming less and less capable of focussing on all tasks,
academic, musical or day to day living, including social interactions.
You seem unaware of the consequences of this and indifferent to any
suggestions, however kindly made, about how you could improve. I am not
trying to make you into some kind of stress puppy who is constantly
"doing", quite the opposite. I would like you to be more in control of
your life so that you could rest properly when things were done.
You tend to skim over the surface of information, rather than seeking in
depth understanding, which as you progress through college and
university will be less and less satisfactory.
In so many things you make a huge effort, but with other things you just
coast along and hope it will be ok. You are very kind and caring, but
find it difficult to show this to others. You find it very difficult to
enter into a sharing of discovery- you either already know it and do not
realise that it is rude to be constantly pointing this out, or you can't
be bothered because you do not see the relevance to your study. For me,
this is particularly difficult. You do know a lot, but you do not
understand as much as I think you should and it stops us sharing stuff
like the information on relativity that I was hoping to go over with you.
I am sorry if these comments are hurtful - they aren't meant to be. It's
just that if I write it down, you are not constantly interrupting to
talk back and I find that incredibly rude and unconstructive. So do most