The energy of a photon is $3\times {10}^{-12}\text{ ergs. }$What is its wavelength in nm ?

$\left(\mathrm{h}=6.62\times {10}^{-27}\text{ erg.sec; }\mathrm{C}=3\times {10}^{10}{\mathrm{Cm}}^{-1}{\mathrm{s}}^{-1}\right)$

$\begin{array}{l}\mathrm{E}=\mathrm{hv}\Rightarrow \mathrm{E}=\frac{\mathrm{hc}}{\mathrm{\lambda }}\mathrm{E}=\mathrm{hv}\Rightarrow \mathrm{E}=\frac{\mathrm{hc}}{\mathrm{\lambda }}\mathrm{E}=3\times {10}^{-12}\text{ ergs }\mathrm{h}=6.625\times {10}^{-27}\mathrm{erg}\cdot \sec \\ \mathrm{C}=3\times {10}^{10}{\mathrm{cm}}^{-{\mathrm{s}}^{-1}}\mathrm{\lambda }=\frac{\mathrm{hc}}{\mathrm{E}}=\frac{6.625\times {10}^{-27}\times 3\times {10}^{10}}{3\times {10}^{-12}}=662.5\times {10}^{-7}\mathrm{cm}=662.5\mathrm{nm},=662\mathrm{nm}\end{array}$