An insulator has optical absorption, which occurs for all wavelengths shorter than $1800\text{Å}$ . The width of the forbidden energy gap for this insulator in electron volts is 

The energy gap between conduction and valence band for an insulator that is forbidden gap is $\mathrm{\Delta E}=\mathrm{hv}$ where E is energy in eV, h is Planck's constant and v is frequency $(\mathrm{v}=\mathrm{c}/\mathrm{\lambda })$ 
$$\begin{gathered} \mathrm{E}=\frac{\mathrm{hc}}{\mathrm{\lambda }}=\frac{\left(6\cdot 64\times {10}^{-34}\right)\times \left(3\times {10}^8\right)}{1800\times {10}^{-10}}\text{ joule } \\ =\frac{\left(6\cdot 64\times {10}^{-34}\right)\times \left(3\times {10}^8\right)}{\left(1800\times {10}^{-10}\right)\times \left(1\cdot 6\times {10}^{-19}\right)}\mathrm{eV}=6\cdot 9\mathrm{eV} \end{gathered}$$