This month marks the birthday – May 10 — of a little-known French genius who gave us an understanding of light that still informs how we see and recreate it.
“Nature does not dread difficulties of analysis” – Augustin-Jean Fresnel
Since light was first studied in the sixth century B.C., arguments had raged about its essence. Some Ancient Greeks considered light to flow in particles, like the atoms that made up all matter. Others thought it to be shimmering waves called eidola. Particle or wave? The debate summoned all of light’s mysteries.
Everyone knew that sound traveled in waves, but light was different. You could hear someone around a corner shouting but you could not see him. A bell in a glass jar fell silent when the air was pumped out, yet it could still be seen. The debate continued. By the 1600s, Rene Descartes was comparing light to tennis balls bouncing and spinning. Then in 1678, the Dutch astronomer Christian Huygens calculated precisely how light — “an infinity of waves” — bent, bounced, and flowed through the ether. Huygens’ wave theory convinced many, but when Isaac Newton held forth in 1703, arguing that light was “corpuscular,” the debate seemed settled.
Throughout the 1700s, Newton’s particle theory reigned supreme. Then in 1801, British polymath Thomas Young conducted an experiment so original, so unsettling, so perfect that it has been called “the single most influential experiment in modern physics.” Suppose light were waves, posited Young. When those waves cris-crossed, they should “interfere” with each other, like ripples in a pond. Where two waves were in sync, their crests boosting each other, the overlapping light should be brighter. But where the crest of one wave met the trough of another, they should cancel each other, just as waves in a pond overlap to still the water. The result would be darkness, or at least dimness.
Young was soon projecting parallel beams of a single color through adjacent slits. There on his wall where the beams overlapped, Young saw the pattern he expected — vertical bars, bright and
dark. Light waves, Young concluded, are “capable of neutralizing or destroying each other, and of extinguishing the light where they happened to be united.” This he called “the general law of the interference of light.” But Young’s theory was mocked. One critic called interference “one of the most incomprehensible suppositions that we remember to have met with in the history of human hypotheses.” Definitive proof of light waves would have to wait another decade. And, given Newton’s cache in England, such proof would have to come from across the English Channel.
Budding scientists at the Ecole Polytechnique in Paris all learned particle theory, yet by 1814, one alumnus was beginning to doubt. “I tell you I am strongly tempted to believe in the vibrations of a particular fluid for the transmission of light and heat,” Augustin-Jean Fresnel wrote his brother. “One would explain the uniformity of the speed of light as one explains that of sound; and. . . why the sun has for so long shined upon us without diminishing its volume, etc.”
Although doubting Newton, Fresnel was much like him. Newton was known as “fearful, cautious, and suspicious.” Fresnel’s colleagues considered him un homme froid (a cold man). Both Fresnel and Newton were plagued by bad health. And like Newton, Fresnel had what he called “a taste for exactitude” that sent his equations sprawling across page after page.
While working as a civil engineer in rural France, Fresnel began studying light. Within a year of taking up his studies, he presented a paper to the Academie des Frances suggesting that light might be made of waves. Newton’s French disciples martialed their defenses. The aging scientists knew Newton to be right on everything — on gravity, on motion, on the calculus, the prism, and the rainbow. Now they were asked to believe that an unknown civil engineer, not yet thirty, who held no teaching or research position, had a better grasp of light than their “philosophic sun.” Such nonsense might go on indefinitely unless put to a contest.
On March 17, 1817, the Academie des Frances announced the challenge. Entrants were to calculate, using the most refined math, how light traveled around an obstruction. Contestants were given eighteen months. Fresnel took a leave of absence from his engineering post.
He labored for months over the math. Christiaan Huygens’ wave theory, devised before Newton wrote his Principia, did not use calculus. Now Fresnel did, using Newton to disprove Newton. The key tool was the integral. An integral, denoted in an equation by an elegant S (∫), measures curves and the areas they sweep out. Integrals calculate a missile’s trajectory, the graceful shapes of seashells, and as Fresnel saw, the motion of a wave. Applying calculus to light waves, Fresnel crafted what are now called the “Fresnel Integrals.” To the novice they look like modern-day hieroglyphics, a lattice of the highest math that sets the mind spinning. To stare at Fresnel’s equations is to feel yourself drawn towards the infinite complexity of the universe. Symbols and signs are stacked on each other like layers of a cake. The whole is far greater than the sum of the parts, and if you stare long enough, Fresnel’s concerto of calculus is both hypnotic and inspiring. To think that this is how light behaves, to know that a single man opened this door to the eternal is to grasp light’s complexity and marvel at human discovery.
On April 20, 1818, the battle of Particle vs. Wave approached its barricades. Using a Latin epigram Natura simplex et fecunda, “Nature simple and fertile,” Fresnel submitted his contest entry. Only one other was received. A panel of judges convened. The committee, made chiefly of “emissionists” supporting Newton, deliberated throughout summer, fall, and into the new year. One emissionist, mathematician Simeon Poisson, did his own calculations. If light beamed at a disc behaved as Fresnel asserted, Poisson argued, it would leave a bright spot in the dead center of its shadow. C’est absurd! Emissionists thought they had trumped Fresnel, but the committee chairman molded a freckle-sized disc and aimed a beam at it. On the wall behind, there at the center of its shadow – the dead center — was a perfect pinpoint of light. Poisson refused to budge, but the committee was convinced.
Eleven months after submitting his entry, Fresnel was declared the winner. Light, it seemed, was a wave, one that could be calculated with a precision that explained all its mysteries.
Fresnel went on to design giant glass lighthouse lenses that beamed light “like a star” and saved thousands of ships. The name Fresnel now graces several optical concepts, including Fresnel diffraction, the Fresnel spot, and Fresnel lenses found in automobile headlights, cameras, projectors, and solar collectors. The fantastic Fresnel integrals are still used to calculate waves ranging from the curves of roller coasters to the lapping of ocean waves simulated in video games. But Fresnel’s most enduring legacy was his outdueling Isaac Newton to prove that light flowed in waves. At least until quantum theory muddled the waters.