larger stars would consequently emit
slower corpuscles.
Stars of different sizes would therefore
have different aberrations; unfortunately,
direct telescope aberration measurements
were not sensitive enough to observe the
predicted variations. Under the corpuscular theory, however, the refraction of
light was interpreted as an increase in
the normal component of the corpuscle’s
speed. Light of different speeds would
consequently have different refraction
angles, and starlight would thus have different degrees of aberration when viewed
through air or dense media.
Arago devised a clever and simple
technique to test this. He glued an achromatic prism to cover one half of the objective lens of a telescope and examined
the deviation in light rays after passing
through the prism. The prism for his
early experiments was a piece of crown-glass and a piece of flint glass fixed
together, with a total angle of roughly
24 degrees. In this manner he could precisely measure the angle of refraction and
deduce the speed of light from it.
The result of Arago’s experiment
surprised him—he found no significant
variation of the speed of light from
star to star. According to Newtonian
relativity, the speed should at least have
been subject to seasonal variations as the
Earth moved towards and away from
observed stars. Though Arago did see
fluctuations comparable to the seasonal
ones, they were completely uncorrelated
with the seasons and dismissed as experimental uncertainty.
Arago came up with a desperate
hypothesis to explain his negative result:
Stars radiate over a broad range of speeds
with uniform intensity, but the human
eye can only detect the rays that lie within a narrow velocity and spectral range.
This was not as completely off-the-wall
as one might think. Infrared light was
discovered in 1800 by William Herschel,
and in 1801 Johann Ritter discovered
ultraviolet light; both types of radiation
are just outside of the visible spectrum.
Arago used these new phenomena as
evidence for his hypothesis, but the
explanation remained unconvincing.
Courtesy of Greg Gbur
Starlight
Deflected
starlight
Prism
Undeflected
starlight
Arago’s speed of light experiment: With a
prism covering half of the objective of a tele-
scope, Arago could measure the angle of
refraction of starlight, and from this deduce
the speed of light.
Some years later, another famous
scientist would intervene to help explain
Arago’s perplexing results. In 1815,
Augustin-Jean Fresnel presented his first
paper on diffraction theory to the Académie des Sciences. The paper so impressed
Arago that the two quickly became
friends. Fresnel further honed his diffraction theory, and in 1818 offered a
revised memoir for the Académie’s prize
problem: a physical explanation for
diffraction. The prize committee was
nonplussed when Fresnel proposed that
diffraction was evidence for the wave
nature of light. Siméon Denis Poisson
observed that Fresnel’s theory led to the
seemingly preposterous conclusion that a
bright spot of light should appear in the
geometrical shadow of an opaque disk.
Arago performed the experiment, found
“Poisson’s spot,” and thus produced the
first dramatic evidence in favor of the
wave nature of light.
Arago was really repaying a favor for
Fresnel, who that same year had rescued
Arago’s perplexing experiment on stellar
aberration. In an 1818 letter, Fresnel
suggested that Arago’s result could be
elegantly explained by using the wave
hypothesis. He reasoned that light, as a
wave, must propagate in a material medi-
um, the “aether,” as sound waves travel in
air. Aether had been discussed for years
by proponents of the wave theory, but
its behavior was treated as independent
of the motion of matter. Fresnel demon-
strated theoretically that Arago’s negative
result could be explained if the aether is
partially dragged along in the presence of
moving matter, by a degree related to the
refractive index.
Greg Gbur ( gjbur@uncc.edu) is an associate
professor of physics who specializes in optical
science at the University of North Carolina,
Charlotte, N.C., U.S. A.
[ References and Resources ]
>> A. Fresnel. “Lettre d’Augustin Fresnel
à François Arago sur l’influence du
mouvement terrestre dans quelques phé-nomènes d’optique,” Annales de Chimie
et de Physique 9, 57 (1818).
>> F. Arago. Biographies of Distinguished
Scientific Men, Longman, Brown, Green,
Longmans & Roberts, London, U.K.,
1857.
>> F. Arago. “Vitesse de la lumière,” OEuvres
Complètes 7, 548 (1858).
>> Skulls in the Stars blog. François Arago:
the most interesting physicist in the world!
http://skullsinthestars.com/2012/01/16/
francois-arago-the-most-interesting-physicist-in-the-world/ 16 January 2012.
