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A parallel beam of light has intensity 4 W m⁻². Find the amplitude of the electric field in the beam.

1. 35.5 N C⁻¹
2. 45.5 N C⁻¹
3. 49.5 N C⁻¹
4. 54.8 N C⁻¹

Correct answer: 54.8 N C⁻¹

Solution

Using I = (1/2) ε0 c E0² with I = 4, ε0 = 8.85*10⁻¹², c = 3*10⁸: E0 = sqrt(2*4 / (8.85*10⁻¹² * 3*10⁸)) = sqrt(8 / (2.655*10⁻³)) = sqrt(3013) ≈ 54.9 N/C, matching about 54.8 N C⁻¹.

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