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La Lettre
© B.Eymann - Académie des sciences
Alain Aspect
Member of the Académie des Sciences, Emeritus Senior
Researcher at CNRS, Professor at the Institut d’optique and at the
École polytechnique
350 years ago, Colbert invited the great scientists of
the time to join the Académie des sciences. One of the
most eminent of them, Christiaan Huygens, born in 1629
and raised in the Netherlands, had studied Descartes,
Pascal and Fermat, and already produced major results
in mechanics, mathematics and astronomy. He accepted
to come and settle in Paris where he would dwell until
Colbert’s death and the revocation of the Edit de Nantes – events that led him to return to his native
land, to spend there the rest of his life.
Emergence of a wave model of light
In 1672, Huygens discovered Newton’s work on the corpuscular model of light. At the time, the major
problem was to find a model that would justify the laws of reflection and refraction independently formulated
by Descartes and Snell. It is easy to understand that a bouncing corpuscle is a good model to describe the
fact that a ray of light is reflected in a symmetrical way with respect to the normal, but the challenge is to
understand why the refracted ray comes closer to the normal when it passes from air to a denser medium,
such as water or glass. Newton explained it by invoking the attraction of the denser medium: the particles
of light are accelerated perpendicularly to the interface, and the trajectory is therefore closer to normal.
But Huygens was looking for a model that would account for all known phenomena and in particular the
phenomenon of double refraction that is observed with some crystals, such as calcite. He discovered in
1678 that a wave model of light would meet this requirement, while Newton’s corpuscular model did not.
Despite its value, Huygens’ wave model, described in full detail in 1690 in his
Treatise on Light
, was
ignored by most scientists for more than a century. If they adopted the corpuscular model, it was because
of the tremendous prestige conferred upon Newton, who had managed to explain the motions of planets
From Huygens’ waves to Einstein’s
photons: strange lights