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A laser beam has intensity $2.1 \times 10^{15} \mathrm{~W} / \mathrm{m}^2$. The amplitude of magnetic field in the beam in approximately is
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4.2 T
$\mathrm{I}=\frac{\mathrm{B}_0^2}{2 \mu_0} \mathrm{C}$
$\begin{aligned} & \Rightarrow \quad 2.1 \times 10^{15}=\frac{\mathrm{B}_0^2 \times 3 \times 10^8}{8 \pi \times 10^{-7}} \\ & \Rightarrow \quad \mathrm{B}_0^2=17.64 \\ & \Rightarrow \quad \mathrm{B}_0=4.2 \mathrm{~T}\end{aligned}$
$\begin{aligned} & \Rightarrow \quad 2.1 \times 10^{15}=\frac{\mathrm{B}_0^2 \times 3 \times 10^8}{8 \pi \times 10^{-7}} \\ & \Rightarrow \quad \mathrm{B}_0^2=17.64 \\ & \Rightarrow \quad \mathrm{B}_0=4.2 \mathrm{~T}\end{aligned}$
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