The electric field in an electromagnetic wave is given by $E=50 \sin \frac{2 \pi}{\lambda}(ct - x)$ N/C. The energy contained in a cylinder of cross-section 10 cm² and length 50 cm along the x-axis is:

5.5 × 10^-12 J

Average energy density of the electromagnetic wave is

$U_{a v}=\frac{1}{2} \varepsilon_{-} \mathrm{E}_0^2=\frac{1}{2} × 8.55 × 10^{-12} ×(50)^2=1.1 × 10^{-8} \mathrm{Jm}^{-3}$

Volume = $10(\mathrm{cm})^2 × 50 \mathrm{cm}=5 × 10^{-4} \mathrm{m}^3$

U = volume × energy density

= $5 × 10^{-4} × 1.1 × 10^{-8}=5.5 × 10^{-12}$ J.