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A plane electromagnetic wave in vacuum has a frequency of 2.0 * 10¹⁰ Hz and an energy density of 1.02 * 10⁻⁸ J/m³. Taking 1/(4*pi*eps0) = 9*10⁹ N m²/C² and c = 3*10⁸ m/s, the amplitude of the wave's magnetic field is closest to:

1. 180 nT
2. 160 nT
3. 150 nT
4. 190 nT

Correct answer: 160 nT

Solution

Using the total (electric + magnetic) energy density u = B0²/(2*mu0), substituting u = 1.02*10⁻⁸ gives B0 ~ 1.6*10⁻⁷ T = 160 nT. The frequency is extra information not needed for the amplitude.

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