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The electric field intensity produced by radiation from a 100 W bulb at a distance of 3 m is E. What is the electric field intensity produced by a 50 W bulb at the same distance?

1. E/2
2. 2E
3. E/sqrt(2)
4. sqrt(2)*E

Correct answer: E/sqrt(2)

Solution

Electric field amplitude E is proportional to sqrt(intensity) = sqrt(P/(4*pi*r²)) ~ sqrt(P) at fixed distance. So E₅₀/E₁₀₀ = sqrt(50/100) = 1/sqrt(2). Hence E₅₀ = E/sqrt(2).

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