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Question

# A plane electromagnetic wave, has frequency of 2.0×1010 Hz and its energy density is 1.02×10–8 Jm−3 in vacuum. The amplitude of the magnetic field of the wave is close to: (14πε0=9×109 Nm2C2)and speed of light c=3×108 ms−1

A
150 nT
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B
160 nT
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C
180 nT
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D
190 nT
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Solution

## The correct option is B 160 nTThe energy density of the electromagnetic wave in terms of magnetic field is given by, U=12B20μ0 ⇒B0=√2×μ0×U From electromagnetic theory, μ0=1c2ε0 =4π×9×109(3×108)2=4π×10−7 ∴B0=√2×4π×10−7×1.02×10−8 ⇒B0=160×10−9 T ⇒B0=160 nT. Hence, option (B) is correct.

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