7.2.2. Spot of Poisson.

A laser beam illuminates a round disk. Behind the disk, in its shadow, a bright spot is visible which is produced by constructive interference of the waves originating from the edge of the disk. This spot is called ‘the spot of Poisson’. It was predicted by Poisson by applying the Maxwell equations. Poisson concluded that the spot proved that Maxwell’s theory was not valid and that light is not a wave phenomenon. However, Arago performed the experiment and saw the spot. The spot of Poisson is therefore one of the first indications that light is a wave-phenomenon. It is also called the spot of Arago or the spot of Poisson/Arago.

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