Retarded time

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According to Maxwell's Equations, electromagnetic waves in a vacuum travel at the speed of light, c. Since c is finite, a photon emitted when t = t₀ takes a certain amount of time to reach an observer located at a distance r from the source, so that the observer only notices it when t = t₁:

$t_1 = t_0 + \frac{r}{c}$

This can be rearranged as:

$t_0 = t_1 - \frac{r}{c}$

The time $t_0 = t_1 - \frac{r}{c}$ is defined as the retarded time and represents the delay between the photon emission and the moment at which it reaches the observer.