A red LED emits light at 0.1Watt uniformly around it. The amplitude of the electric field of the light at a distance of 1m from the diode is: A. 6V/m B. 2.45V/m C. 5.48V/m D. 1.73V/m

Answer: B. 2.45V/m


Intensity of light, I = P/4πr2 = uav×c

uav = 1/2 ∈0E02

\(\frac{P}{4 \pi r^{2}}=\frac{1}{2} \in_{0} E_{0}^{2} \times c\\ E_{0}=\sqrt{\frac{2 P}{4 \pi r^{2} \in_{0} c}}=\sqrt{\frac{2 \times 0.1 \times 9 \times 10^{9}}{1^{2} \times 3 \times 10^{8}}}\)

= 2.45 V/m


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