Solution

Past exam of the mathematics course of the University of Cambridge / 2023 / ib / Paper 1 / 15D / e / Solution

An isotropic source of average power $P$ has intensity
$$
\langle S\rangle=\frac{P}{4\pi r^2}
$$
at distance $r$. For monochromatic vacuum radiation, its electric-field amplitude is $E_0=\sqrt{2\langle S\rangle/(\epsilon_0c)}$.


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