Armand-Hippolyte-Louis Fizeau Biography

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World of Scientific Discovery on Armand-Hippolyte-Louis Fizeau

In the mid-1800s gentlemen scholars, typified by an abundance of both time and money, often dabbled in science to stave off boredom. While most of these "professional amateurs" produced nothing of worth, occasionally one would appear who possessed both true interest and an aptitude for science. Such a man was Armand Fizeau.

Fizeau was born in Paris. His father held the chair of internal medicine at Paris' Faculty of Medicine, which brought the family both social prominence and wealth. Intending to join his father in the field of medicine, Fizeau enrolled at the College Stanislas; however, the harsh climate there did not appeal to the boy, and he soon returned to Paris to study physics at the College de France. During his years as a student he worked under the tutelage of the renowned astronomer and physicist Dominique-François Arago, the director of the Paris Observatory. It was Arago who recognized Fizeau 's potential and steered him in the direction of experimental physics.

In 1839 Fizeau began a collaboration with Léon Foucault. The two young men were particularly interested in the new photography method that had been developed by L. J. M. Daguerre (1789-1851). By making improvements to the daguerreotype process, they were able to obtain very detailed photographs of the sun's surface in 1845. Concentrating further on optics, Fizeau and Foucault studied the effects of interference on light waves and, later, the interference patterns of heat waves. This research helped to further the view that light acted as a wave rather than as a stream of particles.

Years earlier, Arago had suggested that the only way to prove the wave nature of light was to measure its velocity as it passed through both air and water. If light consisted of particles, light would travel faster in water; however, if light acted as a wave, it would travel faster in air. Fizeau and Foucault set out to perform Arago's experiment in 1845, and within a few years they had developed a method for measuring the speed of light terrestrially. Previously, all figures for light's velocity had been obtained through astronomical observation. Unfortunately, the two scientists soon dissolved their partnership over a personal matter, and began conducting their experiments independently.

Fizeau developed a method for measuring the speed of light in air. On the peak of a hill he set up a light source and a spinning gear, arranged so that the light would shine through the gear's teeth. As it spun, the gear would alternately block and unblock the light, so that it would flash. On another hilltop 5 miles (8 km) away he positioned a mirror that reflected the light back to its source. Fizeau spun the gear very fast, so that light passing through one gap in the gear's teeth would travel to the mirror, bounce back, and reenter through the next gap. By using a timer, he was able to determine the amount of time it took light to travel 10 miles (16 km)--the distance there and back.

In 1849 Fizeau arrived at the figure of 195,615 miles (315,000 km) per second--a number slightly higher (about five percent) than that obtained by astronomical means but certainly far more accurate than any previous terrestrial method had yielded. The modern figure for the speed of light is approximately 186,000 miles (299,700 km) per second.

Fizeau's next step was to measure light'svelocity as it passed through water. He began with a single beam of light and split it into two beams, one of which was shone through water while the other passed through air. Then, using another rotating-gear-and-mirror system, he measured and compared the two velocities. It was found that the speed of light was indeed greater as it traveled through air, thus validating the wave theory of light. Fizeau published his findings in 1850, the same year that Foucault announced his own almost identical results.

While conducting experiments on the speed of light, Fizeau became intrigued by the idea of light from a moving source. It had been discovered by the German optician Joseph von Fraunhofer (1787-1826) that the spectrum of light contained many dark lines, called absorption lines. Fizeau hypothesized that if a light source was moving toward an observer, the waves would be pushed together, shifting the absorption lines toward the violet end of the spectrum; if the source was moving away, the light would shift toward the red.

Unbeknownst to Fizeau, this same hypothesis had been advanced six years earlier by Christian Johann Doppler. However, Doppler's theory was flawed, in that it cited red-and blue-shifting as the reason for stars' colors. By merging the work of both Fizeau and Doppler, later scientists achieved a better understanding of the phenomenon. The shifting of light is now known as the Doppler-Fizeau effect. By measuring the amount and in which direction a star's light is shifted, astronomers can determine whether a star is approaching the Earth or receding from it and how fast. Such information has formed the basis of the expanding universe model of cosmology.

Though he never obtained an advanced degree, Fizeau was considered one of the prominent researchers of his time. With the help of Arago he became a member of the Academy of Sciences in 1860 and was elected its president in 1878. He was awarded the British Royal Society's Rumford Medal in 1866, and in 1856 he was the first to receive the Institut de France's Triennial Prize.