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Question:
Here is one that I have been trying to explain for some time but have be unable to find a satisfactory answer: What makes something transparent? I understand about the index of refraction, however, it seems me that if a substance such as silicon can be made into transparent glass, and a substance like petroleum can be made into transparent plastic, and so on, it stands to reason that something like steel could be made transparent as well. Why cannot aluminum be ma transparent, ala Scotty in Star Trek IV?



Replies:
The main reason is the electrons. The electrons in a metal, which give it its electrical conductivity, cause the light to be reflected. Aluminum oxide can be transparent, aluminum can not.


Yup. Glass is not pure silicon, it is silicon di-oxide (a combination of silicon with oxygen). Plastic not the same material as crude oil - they have totally changed the chemical composition and carbon is notorious for being able to do wonderfully different things in different molecules - Aluminum and other elements just do not have that ability. But, so far we have explored only a very limited range of possible combinations of elements - there are undoubtedly wonderful materials that could be made in the future. Just think - there are 92 different elements (that are relatively stable) so the number of materials with equal parts of just 2 elements is 92^2 = 8464, and you can try arbitrary combinations of any of them. The total number even of things in equal combinations would be 2^92 which is an immensely large number. And then preparation technique etc all enters in. This is why the whole field of materials science is so important these days.

Arthur Smith



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