A laser beam has intensity of 4.0 × 10¹⁴ W/m². The amplitude of magnetic field associated with beam i $s _________$ T. (Take ε₀ = 8.85 × 10⁻¹² C²/Nm² and c = 3 × 10⁸ m/s)
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Question

A laser beam has intensity of 4.0 × 10¹⁴ W/m². The amplitude of magnetic field associated with beam i s _________ T. (Take ε₀ = 8.85 × 10⁻¹² C²/Nm² and c = 3 × 10⁸ m/s)

G1 Physics · Accepted Answer

For an electromagnetic wave, the average intensity is I = ½ε₀cE₀². Also, E₀ = cB₀. Therefore, I = ½ε₀c(c²B₀²) = ½ε₀c³B₀². Hence, B₀ = √[2I/(ε₀c³)]. Substitute: I = 4.0 × 10¹⁴ W/m², ε₀ = 8.85 × 10⁻¹², c = 3 × 10⁸ m/s. Thus, B₀ = √{[2×4.0×10¹⁴]/ [8.85×10⁻¹²×(3×10⁸)³]} ≈ 1.83 T. Correct option: D.