Shufflebrain

Physics would be a side show in our times if the logic of waves had been left out of science. And waves might have been left out of science, were it not for Galileo's discovery of the rules of the pendulum. The pendulum taught us how to build accurate clocks. Without a reliable clock, astronomy would be out of the question. And how could anybody contemplate timing something such as the speed of light without a good clock? It was in 1656 that a twenty-seven-year-old Dutchman named Christian Huygens invented the pendular clock.

Huygens wasn't just a back-room tinkerer. His work with the pendulum was the result of his preoccupation with waves. The reader may recognize Huygens's name from his famous wave principle. He had studied the question of how waves spread to make an advancing wave front. Have you ever observed ripples diverging in ever-expanding circles from the point where you drop a rock into the water? If not, fill a wash basin halfway, and then let the faucet drip...drip...drip! Huygens explained how one set of ripples gives rise to the next. He postulated that each point in a ripple acts just like the original disturbance, creating tiny new waves. The new waves than expand and combine to make the next line of ripples, the advancing wave front. A diagram in a treatise Huygens published in 1690 is still the prototype for illustrations of his principle in today's physics textbooks.

Nor is it a coincidence that Huygens, "during my sojourn in France in 1678," proposed the wave theory of light.

[3]

(He didn't publish his

Treatise on Light

for another twelve years.)

We can't see light waves. Even today, the light wave is a theoretical entity. And to scholars in Huygen's times, nothing seemed sillier or more remote from reality than light waves.

But on November 24, 1803, Thomas Young, M.D.., F.R.S., thought it right "to lay before the Royal Society a short statement of the facts which appear so decisive to me..."

"I made a small hole in a window-shutter, and covered it with a piece of thick paper, which I perforated with a fine needle." [sniff!] Outside the shutter "I placed a small looking-glass...to reflect the sun's light, in a direction nearly horizontal, and upon the opposite wall." And with perforated cards in the path of "the sunbeam," Young produced interference patterns and demonstrated, conclusively, the wavy nature of light.

Young's experiment is a laboratory exercise in physics courses today. It involves two baffles, one perforated in the center by a single pinhole, the other with two pinholes in line with each other but off-center. The experimenter places the baffles between a tiny light source and a screen, locating the one with the single hole nearest the light. When light passes through the single pinhole and then through the two pinholes in the far baffle, interference fringes, dark and light stripes, appear on the screen. What happens if we place a finger over one pinhole in the far baffle? Let's let Thomas Young tell us: "One of the two portions [our pinholes] was incapable of producing the fringes alone." Changes in the intensity of the light don't affect the results. Interference fringes require

two

sets of waves.

Interference patterns guarantee waves. But Young's work wasn't immediately accepted by British scientists. In fact, if he had not been active in many other fields (the range of his intellect is astonishing), he might never have been allowed to lay another thing before the Royal Society. Young's critics, according to his biographer, George Peacock, "diverted public attention from examination of the truth."[4]

It's as though a new wavefront starts out at the slit, whether the waves are light or water.

But across the English channel, Napoleon notwithstanding, Young's work found an eloquent and persuasive champion in the person Francois Arago. And by the time Victoria became Queen, even the English believed in light waves.

It wasn't Arago's original research that vindicated and extended Young's theory, however, but that of Augustin Jean Fresnel, with whom Arago collaborated.

Fresnel! When my mind says "Fray-nel!" in poor Ph.D. language-examination French, certain words of Charles Peirce also surface: "These are men whom we see possessed by a passion to learn...Those are the naturally scientific men."[5] Fresnel's work brought him little renown in his lifetime. But optical physicists today use his name as an adjective. For Fresnel demonstrated just what interference is all about.