A plane electromagnetic wave, has frequency of $2.0 \times 1010 \mathrm{~Hz}$ and its energy density is $1.02 \times 10- 8 \mathrm{~J} / \mathrm{m} 3$ in vacuum. The amplitude of the magnetic field of the wave is close to $\left(\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9 \frac{\mathrm{Nm}^2}{\mathrm{C}^2}\right)$ and speed of liaht $=3 \times 10^8 \mathrm{~ms}^{-1}$ )