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 100W of power. The amplitude of the electric field associated with the electromagnetic radiation at a distance of 10m from the lamp will be

• A. $1.3V/m$
• B. $2.68V/m$
• C. $5.36V/m$
• D. $9.37V/m$

Answer 1

The correct option is A $1.3V/m$

Power of the EM waves is equal to the 3% of 100 W i-e, $P=3W$

Intensity at distance of 10 m, $I=\frac{P}{4\pi R^2}=\frac{3}{4\pi (10{)}^2}=.0024W/m^2$

Using, $I=\frac{1}{2}{ϵ}_{o}c|E{|}^2$

$.0024=\frac{1}{2}\times 8.85\times {10}^{-12}\times 3\times {10}^8\times |E{|}^2$

$⟹|E|=1.34V/m$