A Bright Light in the Dark — Augustin-Jean Fresnel
Augustin-Jean Fresnel achieved distinction in science and in public service by remaining true to his principles during unsettled times. After Napoleon Bonaparte’s empire collapsed, France returned to monarchy rule under the brother of Louis XVI, the French king who was executed during the French Revolution. These were dark times, and Fresnel paid a price for his middle-class upbringing and strongly-held political views and religious faith.
As a highly trained engineer, Fresnel found that the work available to him at the start of his career was mundane in nature and offered little promise of advancement. However, Fresnel was also driven by his curiosity in optics and by his desire to contribute to the current scientific research. Members of the Royal Academy of Science were delving into the fundamental nature of light. And, Fresnel was fortunate to attract the interest and support of one of leaders of the Paris scientific community, the astronomer François Arago.
In 1818, the Academy of Science launched a competition for the best explanation of the phenomenon of the diffraction of light. With the encouragement of Arago, Fresnel submitted a mathematical analysis based on the premise that light behaves not as a stream of particles, as hypothesized by Isaac Newton, but as a wave. This ran against the thinking of most scientists. For decades the astronomer Simon Laplace and his followers had advanced the conservative view that the world could be explained in terms of the principles established by Newton centuries earlier. Arago took the anti-establishment view that the world is full of mystery, and the role of science is to discover truths as yet unknown.
Denis Poisson, a protégé of Simon Laplace, was one of the judges of the competition and very critical of Fresnel’s analysis. Poisson pointed out that, if Fresnel was correct, diffraction should produce a bright spot of light exactly at the center of the shadow cast by a circular target. Clearly this could not be true; therefore Fresnel’s analysis suffered from the flawed premise that light behaves like a wave.
Arago set up an experiment to test of Poisson’s assertion of the absurdity of Fresnel’s analysis. The results showed, empirically, that a diffraction does indeed produce a bright spot of light in the center of the circular shadow. Fresnel won the prize, and Arago’s argument that there was still much for science to discover was confirmed. Fresnel continued to work as a member of the national corps of engineers even as he expanded his scientific studies to include the phenomenon of the polarization of light.
In 1809, Fresnel took up the practical problem of how to make lighthouses more effective as a aid to coastal navigation. Fresnel designed a compound glass lens capable of gathering the light produced by a lamp and focusing it into a powerful beacon that could be directed out to sea. Fresnel installed his first lens in the Cordouan lighthouse near the mouth of the Gironde estuary in 1823. This was so successful that Fresnel was promoted to secretary of the Commission des Phares, and he devoted the rest of his life to modernizing and expanding France’s system of lighthouses. Fresnel’s lens quickly became the standard design for lighthouses built around the world.
Augustin Fresnel is one of the 72 engineers and scientists named on the Eiffel Tower.
