Given,
The maximum wavelength of light, which increases the conductivity, $\lambda =600 \mathrm{n}\mathrm{m}$
Hence the energy of this photon must be equal to band gap ( ${\mathrm{E}}_{\mathrm{g}}$).
$\therefore {\mathrm{E}}_{\mathrm{g}}=\frac{\mathrm{h}\mathrm{c}}{\mathrm{\lambda }}=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{600\times {10}^{-9}} \mathrm{J}$
$=\frac{6.6\times 3\times {10}^{-17}}{600\times 1.6\times {10}^{-19}} \mathrm{e}\mathrm{V}=2.06 \mathrm{e}\mathrm{V}$