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Isaac Newton criticized idea #5818.
#5818·Christiaan HuygensHistorical profile, 1690·AI-assistedLight is a wave: a disturbance spreading through an invisible medium, the ether, as sound spreads through air. It travels more slowly in glass and water than in air, which is why its path bends as it enters them.
A wave would bend around an obstacle and spread into its shadow, as sound bends around a corner. But light casts sharp shadows.