Why is glass transparent?

11 October 2009

Question

Why is glass transparent? I’ve heard that glass can be made from sand. How is this given that sand is not transparent, you can’t see through it, but you can see through glass?

Answer

Well if you look very, very carefully at a single grain of sand, especially if you've polished up the surfaces, it is actually transparent. I've looked through lumps of sand, quartz- white sand, through microscopes. As long as it's polished, you can see through it. Sand is intrinsically transparent. The reason why you can't see through it on a beach is because it is in lots of lumps. If you ever looked at the world through a glass or a piece of glass, especially if it's curved, everything looks distorted behind it because when light hits it, it slows down, it bends, it goes around a corner it gets refracted, and the light kind of gets bounced off. Now, if you're looking at something large, you can still make up a picture behind that. But if you've got thousands and thousands of very small glasses, the light would all get refracted off one. It would get refracted offsome others, and all the pictures will get mixed up and mixed up and mixed up, until eventually they get overlaid over one another and it looks white. Chris - So it must be the same phenomenon as snow looking white, but the water it's made from - if you see it in fish tank, is transparent. Dave - Yeah and even in ice it's also transparent. Helen - And then you've got sort of yellow sands, black sands, and the other impurities that are giving it that kind of tinge of a different colour path. Dave - Yeah, you get different rocks in it. Black sands are normally from basalts which is intrinsically black and those just aren't transparent and yeah, a bit of patchy clays and all sorts of things in there.

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