Question

A lamp emits monochromatic green light uniformly in all directions. The lamp is `3%` efficient in converting electrical power to electromagnetic waves and consumes `100 W` of power. The amplitude of the electric field associated with the electromagnetic radiation at a distance of `5` m from the lamp will be:
A. `1.34 V//m`
B. `2.68 V//m`
C. `4.02 V//m`
D. `5.36 V//m`

Answer 1

Correct Answer - b
Average intensity `=P/(4piR^2)=1/2 in_0E_0^2xxc`
or `E_0=sqrt(P/(2pivR^2in_0c))`
`=sqrt(3/(2xx3.14xx25xx8.85xx10^(-12)xx3xx10^8))`
`=2.68Vm^-1`