A red LED emits light at 0.1 watt uniformly around it. The amplitude of the electric field of the light at a distance of 1m from the diode is:
JEE Main 2015 — Physics Electromagnetism
2015mcqmedium
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:
Official previous-year question
Held on 4 Apr 2015 · Verified 6 Jul 2026.
Options
A
7.75Vm−1
B
1.73Vm−1
C
2.45Vm−1
D
5.48Vm−1
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Solution
For a point source of power = P, then intensity at a point at a separation x from the source is, I=AreaPower=4πx2P∵ The average intensity of E.M. wave is given by, I=21cϵ0Eo2⇒E0=4πϵ0cx22P
∵4πϵ01=9×109,P=0.1W,x=1m, c=3×108ms−1
⇒E0=3×108×122×0.1×9×109=6=2.45Vm−1
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