For a sinusoidal plane electromagnetic wave in vacuum having peak electric-field amplitude E₀, the average intensity is:
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Question

For a sinusoidal plane electromagnetic wave in vacuum having peak electric-field amplitude E₀, the average intensity is:

G1 Physics · Accepted Answer

Instantaneous Poynting-vector magnitude is S = EB/μ₀. Using B = E/c, S = E²/(μ₀c) = ε₀cE². For a sinusoidal wave, E = E₀cos(kx−ωt), so = E₀²/2. Therefore average intensity is I = ~~= ε₀c(E₀²/2) = ½cε₀E₀². Correct option: B.~~