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8.Electromagnetic waves
hard
A plane electromagnetic wave, has frequency of $2.0 \times 10^{10}\, Hz$ and its energy density is $1.02 \times 10^{-8}\, J / m ^{3}$ in vacuum. The amplitude of the magnetic field of the wave is close to$....nT$
$\left(\frac{1}{4 \pi \varepsilon_{0}}=9 \times 10^{\circ} \frac{ Nm ^{2}}{ C ^{2}}\right.$ and speed of $1 ight$ $\left.=3 \times 10^{8}\, ms ^{-1}\right)$
A
$180$
B
$160$
C
$150$
D
$190$
(JEE MAIN-2020)
Solution
Energy density $\frac{ dU }{ dV }=\frac{ B _{0}^{2}}{2 \mu_{0}}$
$1.02 \times 10^{-8}=\frac{ B _{0}^{2}}{2 \times 4 \pi \times 10^{-7}}$
$B _{0}^{2}=\left(1.02 \times 10^{-8}\right) \times\left(8 \pi \times 10^{-7}\right)$
$B _{0}=16 \times 10^{-8} T =160 nT$
Standard 12
Physics
