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:

$t\_1\; =\; t\_0\; +\; \backslash frac\{r\}\{c\}$.

This can be rearranged as:

$t\_0\; =\; t\_1\; -\; \backslash frac\{r\}\{c\}$.

The time $t\_0\; =\; t\_1\; -\; \backslash frac\{r\}\{c\}$ is defined as the retarded time and represents the delay between the photon emission and the moment at which it reaches the observer.

See also

• Jefimenko's equations

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