Discussion Forum

A red LED  emits light at 0.1 W uniformly around it. The amplitude  of the electric field  of the light  at a distance of 1m from the diode is

Answer:

Explanation:

Consider the LED  as a point source of light

 Let the power of LED is  P.

 Intensity at r from the source

           $I=\frac{P}{4\pi r^{2}}$  ..........(i)

 As we know that 

            $I=\frac{1}{2}\epsilon_{0} E_{0}^{2}c$              ..........(ii)

 From Eq  (i) and (ii)  , we can write

               $\frac{P}{4\pi r^{2}}=\frac{1}{2}\epsilon_{0} E_{0}^{2}c$

or            $E_{0}^{2}=\frac{2P}{4\pi\epsilon_{0}r^{2}c}=\frac{2\times0.1\times9\times10^{9}}{1\times3\times 10^{8}}$

     or      $E_{0}^{2}=6\Rightarrow E_{0}=\sqrt{6}=2.45V/m$