Out, Damn'd Spot!

Ideas about the nature of light have been around for thousands of years, but until Newton came along in the 17th century most of these attempts were little more than speculation. Newton himself held to the view of light as composed of huge numbers of tiny "corpuscles", or particles, which bounce of mirrors and are absorbed by dark objects, etc. It wasn't a bad idea, really. It explains shadows very well, for instance. If you stand in the way of particles, they can't go through you and so you'll leave a shadow behind you where the light particles have failed to hit.

Particles weren't the only possible explanation, and indeed some of Newton's prism and diffraction experiments are now known to be very well explained by the idea of light as a wave. This wasn't so clear at the time though, because wave behavior can be rather counterintuitive.

A century or so later a scientist named Augustin-Jean Fresnel was poking around with the wave hypothesis and wrote a discussion on what we now call Huygen's principle - the concept that when light encounters obstructions, the edge of that obstruction itself acts as a source of light waves. This is analogous to water waves in a swimming pool. If you're floating in an inner tube and someone does a cannonball near you, the waves produced will reach you. But instead of leaving a wave-free shadow behind you, the waves bend around you as though they had in a way originated with you in response to the original wave from the diver.

This effect isn't always obvious with light, because this diffraction effect is most pronounced when the wavelength of light is comparable to the side of the obstruction. The wavelengths of the water wave aren't too much smaller than you, but the sub-micron light waves are too small to noticeably bend around you under most circumstances. Radio waves, incidentally, are another story. You can listen to the radio pretty well even if there's a building between your car and the radio tower, because radio waves have a wavelength comparable to the building size.

At the time most of the above two paragraphs in relation to light was as yet unknown. So another great physicist named Siméon-Denis Poisson thought Fresnel's idea was self-evidently ridiculous. "If Fresnel's idea is correct, then the edges of a circular obstruction will act as sources of light waves. Most of these will cancel out and produce a shadow behind the object, as expected. But because the path length from the edge to the middle of the shadow is equal no matter where on the edge you start, the cancellation can't happen and there has to be a bright spot right in the middle of the shadow. This is self-evidently bogus."

Well, those weren't quite his words. But it was a good argument either way. He was entirely right about what Fresnel's idea implied. But then Poisson's colleague François Arago decided to actually do the experiment. It's more delicate than it looks because the source of light has to be precisely generated and the circular obstruction has to be very exactly circular to within roughly a wavelength of light. Even then those conditions were achievable, and Arago did the experiment. Oops:

Which was of course great news for Fresnel. If your theory comes up with a ridiculous prediction totally at variance with the previous best idea, and that prediction turns out to be right, it's a pretty good sign you've come up with the right advance. We've had many more advances since then, and now we know that even the wave description is an approximation of the more precise quantum theory of light. But to this day the wave description that Fresnel helped to establish is precise enough for most optical technology. For an idea that started off as a crazy spot, that's not bad.