Wednesday, November 03, 2010

Why light passes through glass

The Longeway book arrived very quickly (2 days) and is a credit to Amazon . Compare this with Waterstone's, who had no copy in any of their London shops, and who said that ordering may take weeks or months, or with the university libraries in London (only UCL library had a copy, but this horribly-designed and uncomfortable building is to be visited only as a last resort).

There is much to say about the book. The introduction is long and as interesting as the reviews suggest. One example, illustrating Longeway's attention to detail, is the way he notices the interesting passage by Aristotle at 88a11. This is in some ways more interesting than the later and better known passage about the lunar eclipse beginning at 89b26. In the case of the eclipse it is theoretically possible for us directly to observe to cause of the eclipse (namely, as he says at 90a24, if we were living on the moon). In the case the transparency of glass, by contrast, it is theoretically impossible for us to observe directly the passage of light through the 'pores' in glass. The passage is also interesting for the insight that some ancient Greek scientists thought that the transparency of glass could be explained through some atomic or molecular theory.

On why glass actually is transparent, see this elementary explanation. It is intended for children, although I didn't follow it that well. It says the reason is that the molecules in liquids are disorganised and random, and so light can pass through them. It cannot pass through solids, because the arrangement of molecules is ordered (I didn't follow this reasoning). Light passes through all liquids, glass is a liquid, therefore light can pass through glass (I did follow this, however).

Note we can express the second reasoning in both Aristotelian propter quid and quia forms, as follows.

Propter quid
Light passes through liquids
Glass is a liquid
Therefore, light passes through glass

Light only passes through liquids
Light passes through glass
Therefore glass is a liquid

My earlier observations apply here as well. Both syllogisms are essentially trivial and hardly count as 'reasoning' at all. The real reasoning involves how we arrive at the (superficially implausible) premiss that glass is a liquid.

1 comment:

Edward Ockham said...

>> Kraxpelax said... Intelligence!

Happy to post a link or two to your blog, but would appreciate an intelligent comment by way of compensation :)