Question

A red LED emits light at $0.1$ watt uniformly around it. The amplitude of the electric field of the light at a distance of $1m$ from the diode is (in V /m)

Answer 1

Intensity $I=\frac{P}{4\pi r^2}=U_{av}\times c..........\left(1\right)$
$U_{av}=\frac{1}{2}{ϵ}_0{E}_0^2................\left(2\right)$

From $\left(1\right)and\left(2\right)$

$\Rightarrow \frac{P}{4\pi r^2}=\frac{1}{2}{ϵ}_0{E}_0^2\times c$
$\Rightarrow E_0=\sqrt{\frac{2P}{4\pi r^2{ϵ}_{0}c}}=2.45V/m$