First slide

Intensity of electromagnetic wave

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 10 m from the lamp will be

Difficult

A

1.34 V/m
B

2.68 V/m
C

5.36 V/m
D

9.37 V/m

Solution

$S_{av} = \frac{1}{2} ε_{0} {cE}_{0}^{2} = \frac{P}{4 {πR}^{2}}$
$\Rightarrow E_{0} = \sqrt{\frac{P}{2 {πR}^{2} ε_{0} c}}$
$= \sqrt{\frac{3}{2 \times 3 .14 \times 100 \times 8 .85 \times 10^{- 12} \times 3 \times 10^{8}}} = 1.34 V / m$

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