Retarded time

Propagation delay of EM radiation (light)

In electromagnetism, an electromagnetic wave (light) in vacuum travels at a finite speed (the speed of light c). The retarded time is a wave emission time, which is a wave observation time subtracted by the wave propagation delay between the emission and the observation, since it takes time for information to travel between an emitter and an observer. This arises due to causality.

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