The amplitude of a light wave which is travelling in a medium can be represented by $A \exp \{i(n \omega x / c-\omega t)\}$, where $n$ is the refractive index and $c$ is the velocity of light in vacuo. Show that if the light is attenuated as it passes through the medium then this can be represented by taking $n$ as complex, i.e. in the form $n+i k$. Express $\varepsilon^{\prime}$ and $\varepsilon^{\prime \prime}$ in terms of $n$ and $k$.