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The Brightest Light On Earth

The Brightest Light on Earth

In 332 BCE, Alexander the Great mapped out a city worthy of his name. On one edge would be the Gate of the Sun, at the other the Gate of the Moon. And just off shore would be a lighthouse.

Any ordinary light atop a tower would have steered ships from dangerous shoals, yet in stone after stone the lighthouse of Alexandria rose. To 100 feet. To 300. As tall as a forty-story building.  Topping out at 450-feet, the lighthouse was, after Egypt’s Great Pyramid, the world’s tallest structure. Arches were added, and balconies and bronze carvings. Huge mirrors atop the tower reflected a blazing furnace, sending out a beam visible for miles. By the dawn of the Christian era, the lighthouse of Alexandria was renowned as one of the Seven Wonders of the World.  As a symbol of brightness, as a security for sailors, as a metaphor for learning, the lighthouse had entered human consciousness.

For more than a thousand years, even as Alexandria itself fell from grace, the lighthouse shone from the city’s shore.  Then came the earthquakes, the first in 956, two more during the fourteenth Lighthouse-of-Alexandriacentury.  The lighthouse went dark; the legends burned on.  Renaissance travelogues claimed the Pharos’ light had once been visible for 100 miles, even five hundred. And the mirror spreading such light had performed reciprocal miracles, so it was said. According to an Arab geographer, “when someone looked in the mirror he could see everything that was taking place in Constantinople, despite the fact that between this city and Alexandria there is the Mediterranean Sea and three hundred leagues.” The Pharos’ mirror was rumored to be still more powerful. A 1550 travelogue claimed that “all ships passing near the column when the mirror was uncovered were miraculously and instantly burnt up.”

Such legends made the Pharos the model for lighthouses throughout the Western world.  None was as tall, but all burned with a fire at the top.  The fire, often backed by mirrors, sent beams sweeping a few miles out to sea, hardly an improvement over the wondrous Pharos.  Then in 1820, a French physicist, frail and fascinated with light, got busy.

FresnelAugustin Jean-Fresnel knew more about light than anyone else on earth.  He had already disproven Newton’s particle theory, upending more than a century of certainty about light.  Continuing to study in his small laboratory near the English Channel, he had crafted integrals we still use today, the Fresnel Integrals that calculate exactly how light travels.  Who was more qualified to build a better lighthouse?

By the 1820s, nearly 200 ships ran aground each year on French shores. Still, sailors by the hundreds drifted toward France’s western flank. There the river Gironde led to the city of Bordeaux whose wines the world demanded. As the coast loomed, the search began for the lighthouse — le phare – at the river’s mouth.

The Cordouan Light was one of Europe’s oldest and tallest. Perched on an outcropping a day’s sail north of Spain, Cordouan was “the Versailles of the sea.” In regal French Cordouan schematicstyle, the ten-story tower featured a first floor bedroom readied for the king, a second floor chapel, and abundant statues throughout. The light, however, was no stronger than when it had first shone in 1611. Back then the 223-foot structure had been topped by a bonfire, but in recent years, keepers had experimented with lamps and mirrors whose puny light had sailors begging France to make this phare worthy of its namesake, Alexandria’s Pharos. Napoleon established a lighthouse commission but it rarely met. Complaints went unheeded. The Cordouan light remained a flickering candle. Then Fresnel began studying the problem.

The problem was reflection, he concluded. Even the largest parabolic mirror placed behind the strongest lamp reflected less than half its wayward light. Applying his elegant integrals and his “taste for exactitude,” Fresnel calculated how to capture light by refraction rather than reflection. Within months, glass factories in Paris were turning out prisms three inches thick and twenty inches long. Following Fresnel’s design of “lenses by steps,” glassmakers assembled polygonal plates two feet across, each ribbed with crystalline circles that focused light from all directions. By April 1821, Fresnel’s lens, a twelve-foot tall glass beehive, was ready for testing. Placed atop the Royal Observatory south of Paris’ Luxembourg Gardens, the lens shot a beam over the Seine, and beyond the hills of Montmartre. Within a year, several hundred pounds of prisms and glass plates were shipped around the cliffs of Brittany and landed at the Cordouan light. Fresnel, though sometimes coughing up blood, spent an entire spring on the wind-whipped outcropping at the river’s entrance.

On July 25, 1823, the dark and dangerous coast of France shone with the brightest light yet made by man. The sweeping white spear, easily visible to the horizon, was also seen by sailors atop mastheads more than thirty miles out to sea. Two years later, British sailors saw a Fresnel light beaming over the English Channel “like a star of the first magnitude.” Fresnel soon succumbed to tuberculosis, dying at age thirty-nine, but his light lit coasts from Sweden to Spain to New Jerseyfresnel light


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