Dominique François Jean Arago

1786-1853

French Astronomer and Physicist

Dominique Arago was a French astronomer and physicist who, in his scientific career, made important discoveries in the areas of optics, electromagnetic radiation, and served as director of the Paris Observatory. He was also politically active, serving as Minister of War and Marine for the provisional government following the 1848 revolution.

Arago was born in Estagel, France, and studied at the École Polytechnique in Paris, where he became a professor of analytical geometry at the age of 23. Arago's first major contributions to science came in 1811 when, working with Augustin Fresnel (1788-1827), he discovered that two beams of light that are polarized in perpendicular directions do not interfere with each other. This curious result, difficult to explain if light was composed of particles, suggested strongly that light consisted, instead, of waves and had a large impact on the study of light for many years. Another of Arago's triumphs involved verification that a bright spot of light does indeed appear in the center of the shadow cast by a circular obstruction (such as a disk). This was predicted by Fresnel in a paper on the diffraction of light, assuming that light consisted of waves and not the particles conjectured by Isaac Newton (1642-1727). Roundly criticized by Siméon Poisson (1781-1840) for this prediction, Arago showed the effect to be real, further helping to confirm the wavelike nature of light.

Later, in 1838, Arago suggested that the velocity of light might be slower as it passes into denser media, such as water or glass. He was unable to perform this experiment due to failing eyesight, but Armand Fizeau (1819-1896) and Jean Foucault (1819-1868) carried out the experiment in 1850 and proved Arago correct. This principle helps to explain the refraction of light in various media such as lenses and diamonds.

Arago was also active in other areas of physics. In particular, he discovered the phenomenon of magnetic induction by showing that a rotating copper disk will deflect a magnetic compass needle suspended above it. This principle is widely used in many electrical generators and motors today. He also worked with the French physicist Jean Baptiste Biot (1774-1862) to accurately measure arc lengths on the earth, proposing this as the basis for standardizing units of length in the metric system.

In astronomy, Arago's field of primary interest, he was no less active. His best-known discovery is of the sun's chromosphere, the lower atmosphere just above the visible surface of the sun. He showed it to be composed chiefly of hydrogen gas, now known to be the primary constituent of the sun and all other stars. Later in his career, he inspired his student Urbain Leverrier (1811-1877) to investigate some irregularities in the orbit of Uranus. These investigations eventually led to the discovery of the planet Neptune, the first planet to be discovered by mathematical calculation and prediction.

Later in his career, Arago became increasingly involved in politics, first as director of the Paris Observatory and secretary for the Académie des Sciences, and later as an active participant in the Revolution of 1830. Named the Minister of War and Marine by the provisional government after the 1848 revolution, Arago was responsible for eliminating slavery from the French colonies and participated in extending universal suffrage to adult French men.

During his career Arago was elected a Fellow of the Royal Society and was awarded the Copley Medal by that body in 1825. He is remembered by Crater Arago on the moon and by a commemorative plaque on the Eiffel Tower. Arago died in 1853 in Paris, shortly after seeing experimental confirmation of his predictions about light's velocity changes in different materials.

P. ANDREW KARAM