The French Captain’s Discovery – Polarized Light

It may not be sunny where you are reading this, but you’ll need sunglasses to continue.  Dark shades in hand, study the screen in front of you, whether laptop, desktop, or mobile.  Now put on your sunglasses.  Whoa, where did the light go?  Try it again.  Without the glasses, you can read these words.  With the glasses, the screen dims or, depending on how much you paid for those glasses, goes dark.  What is happening?  The puzzle of “polarized light” owes its solution to a dapper French captain in Napoleon’s army, a contest of cutting-edge physics, and little crystal found on the coast of Iceland.

Working alone in his room in Paris, Etienne-Louis Malus (1775-1812) used small crystals and complex calculations to prove that light is polarized. Credit: Wikimedia Commons

Working alone in his room in Paris, Etienne-Louis Malus (1775-1812) used small crystals and complex calculations to prove that light is polarized. Credit: Wikimedia Commons

At the dawn of the 19th century, Isaac Newton was the oracle of all things pertaining to light.  In the 100 years since Newton had published The Opticks, light had been the talk of Europe.  Bursting from prisms, light’s lovely colors delighted children and adults.  Poem’s eulogized light’s beauty.  And its avatar, the great Newton, was hailed as “our philosophical sun.”  But although light was widely celebrated, it was rarely studied.  The science of optics, trapped behind Newton’s shadow, made little progress in the 1700s.  If there were anything more to be learned about light, surely the great Newton would have found it.

Throughout the so-called Enlightenment, only a handful of scientists dared to challenge Newton.  Light had to be made of particles because Newton said it was.  The few who considered light a wave had a lot of explaining to do.  Yet some were troubled by an anomaly that had also troubled Newton.  The puzzle centered on what happened to light when it passed through the crystal known as Iceland spar.

Legend had long held that the Vikings used a “sunstone” to steer their ships on cloudy days. A sailor held this crystal to the sky, turned it ninety degrees and, using some directional property of sunlight, found his way home. Perhaps. But the rest of Europe did not discover Iceland spar until the 1660s when a traveler brought several chunks back to Denmark. Many marveled at the crystal. When placed on a printed page, it doubled each letter, making ghostly words hover.  Then someone noticed that the spar split candlelight, sending two beams in different directions.  Newton soon got word and made his own examination. In The Opticks, he noted how “that strange substance, Island (sic) Crystal” divided beams.  His prisms split white light into colors, yet this spar divided light without making a spectrum.  Shining light through two crystals, Newton noticed yet another oddity. If their broad faces were parallel, the second spar also split the first’s beams. But turn one spar 90 degrees and it split one beam but not the other, rather like the sunlight the Vikings had spotted through their sunstones.  Light, Newton concluded, must not be symmetrical.  “Every Ray of Light has therefore two opposite Sides.”

The little crystal, Iceland spar, baffled Isaac Newton and others by splitting light into two beams.  The spar and its anomalies ultimately led to the downfall of Newton's particle theory of light. Credit: Bruce Watson.

The little crystal, Iceland spar, baffled Isaac Newton and others by splitting light into two beams. The spar and its anomalies ultimately led to the downfall of Newton’s particle theory of light. Credit: Bruce Watson.

Iceland spar challenged Newton in ways no scientist dared.  Wave theorist Christiaan Huygens was equally intrigued.  A mathematician on par with Archimedes, Huygens devoted an entire chapter in his 1690 “Treatise on Light” to Iceland spar.  “Amongst transparent bodies,” the Dutch astronomer wrote, “this one alone does not follow the ordinary rules with respect to rays of light.”  The bulk of Huygens’ treatise is coldly clinical, yet he found the spar’s tricks “marvelous.”  He calculated the angles of waves passing through the spar, showing how a crystal could have two refraction indexes, cleaving one beam into two. Yet when pondering why a second spar split the “ordinary” but not the “extraordinary” ray, Huygens was stumped.  Others made their own investigations of Iceland spar, but came away as baffled as Newton and Huygens.  The mystery was finally solved by that dapper French captain.

Etienne-Louis Malus is hardly a household name yet hardly a household is without a device he made possible, including the screen you’re reading.  Born in Paris in 1775, Malus became fascinated with light while studying at the Ecole Polytechnique.  When Napoleon, in 1798,  yanked dozens of young engineers from classrooms to join his Egyptian campaign, Malus was one of the reluctant recruits.  With his dark, curly hair, mutton chops, and stiff epaulets, he cut a dashing figure, yet he was  appalled by war.  Contracting bubonic plague, he was shipped to the pesthouse in Jaffa, then back to Cairo. There he wrote in his journal of “the tumult of carnage…the smell of blood, the groans of the wounded, the cries of the conquerors…”  Somehow surviving, he was sent back into battle. Seeking hope amid misery and slaughter, he turned to light.

Late into the night, in a palm-thatched tent on the sands of the Nile Delta, this brilliant but embattled captain studied light.  His tent glowed as he shifted candles and mirrors, calculated angles, and kept his spirit alive even as his body struggled with dysentery and other diseases.  When finally sent back to Europe, Malus supervised engineering projects while continuing his experiments.  By 1807, suffering what would now be called Post-traumatic Stress Disorder, he was holed up in his room near the Luxembourg Gardens.  There he became enthralled by Iceland crystals.  Beaming light through them, he noticed yet another startling trait.  When he sent a beam through the spar, it split in two, as Newton and others had noticed.  Yet if the beam struck the spar at a precise angle — 52 degrees, 54 minutes — the light shot straight through, without splitting!

Malus soon learned of a contest at Paris’ Academie des Sciences.  Members of that prestigious body loved nothing more than to quarrel, bicker, and compete to prove scientific principles.  The latest contest required entrants to explain and calculate the split rays of Iceland spar. Determined to win the contest, Malus spent a year obsessed with light.  “I live here like a hermit,” he wrote. “I pass whole days without speaking a word.”  In a room littered with candles and crystals, he etched a copper sheet with a scale in millimeters, then set a spar on the copper to measure each angle of each double image. Blending the law of refraction with some advanced algebra, he piled equation on equation.  “No one before had carried the use of intricate algebraic formulas in conjunction with experiment to such a high art,” observed Jeb Z. Buchwald, historian of science at Cal Tech.

In December 1808, Malus submitted his contest entry.  Academie elders, though loyal to Newton, could not refute Malus’ math, which relied heavily on Christiaan Huygens’ controversial wave theory.  Malus won the contest.  It was Huygens’ waves, not Newton’s particles that explained Icelandic spar, but the embattled soldier was not done with his crystals.

One bright afternoon, Malus sat in his room near the Luxembourg Gardens. Spotting sunlight glinting off the windows of the Luxembourg Palace, he lifted a spar to his eyes.  He expected the sun’s reflection to be doubly refracted, yet through the crystal he saw just a single image.  That evening, still peering through spars, Malus studied a candle’s reflection in water.  Stooping to adjust his angle of incidence, he looked, checked, looked again.  At the spot where candlelight struck water at 36 degrees, the double refraction turned to single. Reflected off glass or water, Malus concluded, light changes.

Consulting his trig tables, this time not the sine function but the cosine, Malus concluded that, as Newton had suspected, light has “sides.”  Malus then coined a term we still use — “polarized light.”  Light was indeed asymmetrical.  To test his theory, Malus built an elaborate device with rotating mirrors above parallel axes. Now he could bounce light at any angle. A little more math — “the cosine squared of the planar angle” — a bit more insight — if you rotate a spar ninety degrees the “ordinary” ray behaves like the “extraordinary” and vice-versa — and Malus proved the principle of polarization.  Light did not resemble tennis balls, as his fellow Frenchman Rene Descartes had written, but was more like a football (American) that wobbled end over end or, when refracted at certain angles, flew in a perfect spiral. Light beams passing through Iceland spar or reflected off water or glass became polarized, their “sides” aligned.

David Brewster (1781-1868) advanced the study of polarization, finally finding Brewster's Angle, the exact angle at which light is polarized when reflecting off certain surfaces. Credit: Wikimedia Commons.

David Brewster (1781-1868) advanced the study of polarization, finally finding Brewster’s Angle, the exact angle at which light is polarized when reflecting off certain surfaces. Credit: Wikimedia Commons.

Malus published his discovery in 1809.  The following year, he published his calculations of double refraction through Iceland spar.  One can only imagine what other discoveries awaited this genius, but the horrors of war soon caught up with him. Etienne-Louis Malus died in 1812, at age thirty-seven, of complications from diseases contracted in Egypt.  Though his name is known only to physicists, every decent pair of sunglasses filters polarized glare, and every flat screen in a TV, laptop, or mobile device uses polarized light to brighten and darken its pixels. Others would soon extend Malus’s discovery.  In 1815, the British physicist David Brewster devised a formula to compute the angle at which light polarizes when reflected off different surfaces.  Brewster’s Angle is calculated by using the refractive index of the material and the light’s wavelength.  Brewster’s work enabled later discoveries of chromatic, circular, and elliptical polarization, all paving the way for the light of the twenty-first century, including your screen.  You may put your sunglasses away now.


Author photoBruce Watson is the author of Light: A Radiant History from Creation to the Quantum Age (Bloomsbury, February 2016).  The book traces humanity’s evolving understanding and control of light, starting with creation myths, then moving into scripture, philosophy, architecture, Islamic science, art history, poetry, physics, and quantum physics.

Watson’s previous books include Freedom Summer, Sacco and Vanzetti, and Bread and Roses.  Watson’s work has appeared in the Boston Globe, the Los Angeles Times, American Heritage, the Wall Street Journal, the Washington Post, Yankee, Reader’s Digest, and Best American Science and Nature Writing 2003.

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One thought on “The French Captain’s Discovery – Polarized Light

  1. Reblogged this on FLUOTEC and commented:
    The little crystal, Iceland spar, baffled Isaac Newton and others by splitting light into two beams. The spar and its anomalies ultimately led to the downfall of Newton’s particle theory of light. Credit: Bruce Watson.

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