Q: 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

Sol: Average Intensity $\large = \frac{P}{4 \pi R^2} = \frac{1}{2}\epsilon_0 E_0^2 c $

$\large E_0 = \sqrt{\frac{P}{2 \pi R^2 \epsilon_0 c }} $

$\large E_0 = \sqrt{\frac{3}{2 \times 3.14 \times 25 \times 8.85 \times 10^{-12} \times 3 \times 10^8 }} $

= 2.68 V/m