Question

A red LED emits light at $0.1W$ uniformly around it. The amplitude of the electric filed of the light at a distance of $1m$ form the diode is:

• A. $1.73V/m$
• B. $2.45V/m$
• C. $5.48V/m$
• D. $7.75V/m$

Answer 1

The correct option is B $2.45V/m$

Intensity: $I=\frac{P}{4\pi r^2}$

But, $I=\frac{1}{2}{ϵ}_0{E}_0^{2}C$

$\Rightarrow \frac{1}{2}{ϵ}_0{E}_0^{2}C=\frac{P}{4\pi r^2}$

$\Rightarrow E_0^2=\frac{2P}{4\pi {ϵ}_0{r}^{2}C}=\frac{2\times 0.1\times (9\times {10}^9)}{1^2\times 3\times {10}^8}=6$

$\Rightarrow E_0=\sqrt{6}=2.45V/m$