Tuesday, March 1, 2011

Good stripes, bad stripes

Stripes have been viewed in a variety of ways through time. I would have thought that as soon as people invented the loom, stripes would have followed. Stripes must be one of the easiest patterns to make – yet medieval western Europe shunned them.

Stripped clothing was considered at best demeaning and at worst downright diabolical. On the other hand, dots, discs, stars, rings and other simple repeating patterns were good – even viewed as expressing something majestic. This distinction between good and bad patterns was even applied to the animal world; horses were good and zebras were bad. Fortunately, our views about stripes has morphed with time and I can sleep in striped pajamas without worrying about my soul.

Although stripes can't tell us anything about the wearer's moral character, they can tell you what something is made of – even from a distance. Here is a rough idea how it is done (yes it's another optical trick).

Remember Newton's classic experiment where he shone light through a prism and got a rainbow coloured spectrum? If you look really, really closely at the spectrum you can see hundreds of irregularly spaced, thin, dark stripes, which is exactly what the German scientist, Joseph von Fraunhofer, did in 1814. Today, we know more than 30,000 of these lines exist in the sun's spectrum – but what are they?

Elements, like oxygen, helium and the others on the periodic table, are fundamental. They can't be broken down into smaller parts without taking extreme measures like using a super-colliders. If you shine a light (assuming this light gives off a perfectly continuous spectrum) through a gas of an element, then let the light go through a prism the resulting rainbow will have dark stripes in it. These stripes are called absorption lines and are unique to the element. So the stripes from helium will look different that the stripes from nitrogen. This means that, an element can be identified from its stripes alone.

So all those stripes in the spectrum of the sun tell us what the sun is made of – without having to go there.

Universe, 5th edition by William Kaufmann and Roger Freedman, W.H. Freeman and Company, New York, 2000.
The Devil's Cloth: a history of stripes and striped fabric, by Michel Pastoureau, Columbia University Press, 1991

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