Physics - EM Waves Question with Solution | TestHub

The average total energy density of the electromagnetic wave (in vacuum) is twice the average energy density associated with the electric field: $⟨u⟩=2U_E$. Given $U_E=1.0\times 10^{-7}{\text{ J/m}}^3$, so $⟨u⟩=2\times (1.0\times 10^{-7})=2.0\times 10^{-7}{\text{ J/m}}^3$.

The intensity of the electromagnetic wave is given by $I=⟨u⟩c$, where $c$ is the speed of light in vacuum ($3\times 10^8\text{ m/s}$). $I=(2.0\times 10^{-7}{\text{ J/m}}^3)\times (3\times 10^8\text{ m/s})=60{\text{ W/m}}^2$. For a perfectly absorbing surface, the radiation pressure $P=I/c$.$P=(60{\text{ W/m}}^2)/(3\times 10^8\text{ m/s})=20\times 10^{-8}{\text{ N/m}}^2=2\times 10^{-7}{\text{ N/m}}^2$. Comparing this with $N\times 10^{-7}{\text{ N/m}}^2$, we find $N=2$.