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 :

• A. $6V/m$
• B. $2.45V/m$
• C. $5.48V/m$
• D. $1.73V/m$

Answer 1

Answer: (B)

$2.45V/m$

Sine we know that, intensity $I=\frac{P}{4\pi r^2}$
Also, $I=\frac{1}{2}{\epsilon }_0{E}_0^2\times c$
$\frac{P}{4\pi r^2}=\frac{1}{2}{\epsilon }_0\times E_0^2\times c$
$E_0^2=\frac{2P}{4\pi {\epsilon }_0{r}^{2}c}$
$\frac{2\times 0.1\times 9\times {10}^9}{1\times 3\times {10}^8}=6$
$\therefore E_0=\sqrt{6}=2.45V/m$