Built on Facts

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.


  1. #1 Ethan Siegel
    September 23, 2009

    Good stuff; the Poisson spot is one of my favorite examples of wave behavior!

    But give Christiaan Huygens his due. (Which is a lot more than having Fresnel’s principle named after him.) His development of wave theory (and his insistence that it applied to light) not only predates Fresnel, it predates Newton. And it’s right.

  2. #2 Nathan Myers
    September 23, 2009

    While we’re giving optickal due, let’s mention al-Haytham’s 11th-century book “Optics” that includes the first exposition of the scientific method, later noted and promoted by Bacon.

  3. #3 pft
    September 24, 2009

    A similar experiment was actually done by Thomas Young years earlier, in 1802, which established the wave nature of light. He was ignored and ridiculed, as an Englishman, because folks were offended that anyone would challenge Gods science, I mean, Newtons science. You see, it was not only religion that suppressed science, Young was forced into scientific limbo much like Galileo. Sometimes the greatest obstacle to science is other scientists who seek to protect old ideas they are most comfortable with, and/or to protect icons like Newton and Einstein.

  4. #4 ppnl
    September 24, 2009

    The coolest thing that happens in science is when it finds itself suddenly and completely wrong. Most philosophies cannot handle being wrong. But if your interest is reality then being wrong is a possibility you must face. As others have pointed out this is a hard thing even in science. But what else even tries?

  5. #5 Nick
    September 24, 2009

    The greatest discoveries are proclaimed not by shouting ‘Eureka’ but by muttering ‘Hmm, that’s funny.’

  6. #6 andy
    September 24, 2009

    Bear in mind that Arago did the experiment without lasers, because they hadn’t been invented. That makes it somewhat harder. Have you got a link to a description of Arago’s actual setup, rather than the “how to do it with a laser” version you’ve linked in the post?

  7. #7 Uncle Al
    September 24, 2009

    Project a focused slide. Remove the slide to get white light. Interpose an unopened Coke can with its long axis normal to the screen. A very credible Fish spot will appear at the center of its circular shadow.

  8. #8 penis büyütücü
    September 26, 2009

    roject a focused slide. Remove the slide to get white light. Interpose an unopened Coke can with its long axis normal to the screen. A very credible Fish spot will appear at the center of its circular shadow.

  9. #9 Thony C.
    September 27, 2009

    I have written a chronology of the wave theory here

  10. #10 Robert E. Harris
    September 30, 2009

    We have a set of tornado alarm sirens around our town. They are mounted on tall poles and have a large horn to direct the sound outward. The horn rotates slowly in a horizontal plane as the siren sounds. They are tested once a week. Once, I was near one of these as the test started.
    As the horn rotated I heard a louder sound as the horn turned toward me, and then decreasing volume as it turned away, until it was nearly directed away from me. Then the sound grew louder for a few moments and then decreased again, until the horn turned in my direction again: a sonic analog of Arago’s spot.

    We sometimes forget that nice examples of wave phenomena can be produced with sound waves, as the wave lengths are of the order of human dimensions.

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