Tuesday, 31 December 2013

Well tell me then

Well that really worked a treat. There’s a thingy on my blog that tells me how many people are looking at it, (though not, you will be relieved to hear, who they are,) and so now I know that putting a poem in, or just mentioning poetry, causes the readership to plummet.

The trouble is the absence of what is now called ‘Feedback’. In spite of repeated invitations to do so, with the honourable exceptions of you, Dimitri, and you, Jane, no-one ever writes in to say ‘Well that was a load of old codswallop’ or ‘Gosh that was fascinating.’ It’s like the stand-up comedian’s nightmare: the sullen, unresponsive, indifferent audience. Far better the shouts of ‘Boo! Get off!’ and the bad eggs and rotten tomatoes hurled from the gallery.

So if you won’t tell me what you’d like me to write about, I’ll just have to guess. Today’s subject is the corpuscular theory of light. (I can almost hear the eager chorus; ‘Great! At last! A subject of wide general interest!’ Well tell me then.)

            The atoms of Democritus
            And Newton’s particles of light
            Are sands upon the Dead Sea shore
            Where Israel’s tents do shine so bright.

That’s William Blake. (I think I’ve got it right; I’m quoting from memory.) John Keats too, who with his medical training should have known better, thought Newton had destroyed the rainbow by analysing it. In Newton’s time natural philosophers (what we should now call scientists) argued over whether light was made of particles or was a wave motion. The argument continued until the twentieth century, when some peace-loving person came up with the notion of a ‘Wavicle’. Sometimes light is a wave, sometimes it’s a stream of particles. Actually of course it’s neither; light waves or particles have no more ‘real existence’ than do lines of longitude. We would be all at sea without lines of longitude, (if you work for the BBC you must pronounce it ‘Longditude’, just as you must pronounce ‘Antarctica’ as ‘Antartica’ and ‘February’ as ‘Febury’ and ‘Medicine’ as ‘Med-sun’. Sorry; end of irrelevant rant) but I don’t think transatlantic liner passengers were ever disturbed by the ship’s bumping over a line of longitude like a train bumping over the points. Scientific theories are just fantasies, but fantasies with explanatory value.

Newton, rather to the surprise of later theoretical physicists, went for the ‘Corpuscular’ (Particle) theory of light: light as a stream of little balls, bouncing off things or going through them and entering our eyes. The idea makes a lot more sense than it might seem to, in fact it’s helpful in understanding how magnifying glasses, microscopes, and even electron microscopes work.

So, on the corpuscular theory of light, how does a magnifying glass or a microscope work? The best analogy I can think of today (I’m not feeling too brilliant but nor it seems is anyone else in the island; you could hardly get into the doctor’s waiting room this morning so I gave up and went away) is a slide projector, of the sort with which people used to bore their dinner guests rigid (‘That’s me and Mavis coming out of our hotel’) before everyone had computers. Suppose you want to examine the fine structure of something or other. You take a very thin slice of it (microscopists use microtomes, which have blades from cut-throat razors, or pieces of glass or even diamond) and put it in the slide slot. Then you bombard it with particles of light (the projector bulb) and put a screen in front. Where the light particles hit something solid in the sample, none of them reaches the screen. Where they get through gaps, they do. (Duh.) So on the screen is a blown-up representation of the sample.

Now there’s a limit to the blowing-up. What happens if we want to see the really fine structure, what is called the ultrastructure, where the gaps between the solid bits are smaller than the particles of light? Well you can’t. It would be like expecting tennis balls to go through the net. That’s that. Microscopes just can’t go beyond a certain magnification. For a long time it was thought we could never never see the really fine structure of cells and so on; we made it all up and amazingly (as it turns out) got a lot of it right.

But hold on: tennis balls? Suppose we were to use ping-pong balls? Not much use for tennis, but they would go through the net surely? And that’s where the electron microscope comes in handy. Instead of great big ‘Photons’ (as particles of light are called) we use electrons, which are very tiny indeed. So tiny that in order to see them you would need …  


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