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350
YEARS
OF
SCIENCE
67
Siméon Denis Poisson
(1781-1840)
The Poisson’s bright spot: Fresnel theory predicts
the existence of a bright spot in the middle of the
shade. The observation of that counter-intuitive
feature played an important role in the success of
Fresnel wave-theory of light
Screen
Ball
Shade
© From B.Eymann - Académie des sciences
die-hard Newtonians and a sophisticated mathematician, discovered a
surprising consequence of Fresnel’s equations: if a light source is placed on
the axis of a circular obstacle that blocks light, the theory predicts there will
be a bright spot behind the screen, at a place where common sense tells us
utter darkness reigns.
Does it not show that
Fresnel’s theory is
absurd? This is when
the events took a
surprising turn that
the history of physics
still
remembers:
Arago decided that an
experiment should be
conducted, and, to the
amazement of Poisson and the Newtonians, the bright spot was indeed observed. The story has it that
the Académie then shifted in favour of the wave theory, awarding Fresnel the prize of the competition and
soon admitting him as a new member.
Some historians of science claim that the story has been embellished and that the conversion to wave
theory was far from being so massive. As a matter of fact, explained for example Dominique Pestre (citing
John Worrall), Poisson was dazzled by the elegance and mathematical consistency of Fresnel’s theory,
and was waiting only to welcome him at the Académie, while being at the same time not really convinced
Newton’s optics should be rejected. One cannot fail to notice, indeed, that the followers of the corpuscular
theory remained active in the subsequent years until a new episode would clear the debate. Arago –
him again – once more played a key role in 1838 when he proposed a crucial experiment to settle the
argument between Newton’s corpuscular theory, then called the “system of emission”, and Fresnel’s wave
theory, called the “system of undulations”. All it takes, he explained, is a comparison between the speed
© FALKENSTEINFOTO - Alamy