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A red LED emits 0.1 W of light uniformly in all directions. Find the amplitude of the electric field of this light at a distance of 1 m from the diode.

1. 2.45 V/m
2. 5.48 V/m
3. 7.75 V/m
4. 1.73 V/m

Correct answer: 2.45 V/m

Solution

Power spreads over a sphere: I = P/(4 pi r²) = 0.1/(4 pi (1)²) = 7.96*10⁻³ W/m². Average intensity in terms of peak field: I = (1/2) c epsilon0 E0². Solve for E0 = sqrt(2I/(c epsilon0)) = sqrt(2*7.96*10⁻³/(3*10⁸ * 8.85*10⁻¹²)) = sqrt(2*7.96e-3/2.655e-3) = sqrt(6.0) ~ 2.45 V/m.

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