Shadow
The dark region formed behind an opaque object where Light is blocked, divided into the fully blocked umbra and the partially blocked penumbra depending on the size of the light source.
In 1818, Siméon Denis Poisson argued that if light were a wave, a bright spot should appear at the center of a circular object's shadow — which he thought was absurd. But when Arago actually ran the experiment, the bright spot appeared, proving light's wave nature.
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