An observer stands 90 cm from a point source of light with power 250 W. What is the rms value of the electric field at the observer's position?

48 V/m
96 V/m
40 V/m
80 V/m

Correct answer: 96 V/m

Solution

Intensity I = 250/(4*pi*0.81) ≈ 24.56 W/m². Using I = epsilon0*c*E_rms²: E_rms = sqrt(24.56/(8.85e-12 * 3e8)) = sqrt(24.56/2.655e-3) ≈ sqrt(9250) ≈ 96 V/m.

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