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

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

$E=3\times {10}^{-12}ergs\lambda =?nm$

$E=\frac{hc}{\lambda }$ $\Rightarrow \lambda =\frac{hc}{E}$

                           $=\frac{6.6\times {10}^{-27}\times 3\times {10}^{10}}{3\times {10}^{-12}}cm$

                          $=6.6\times {10}^{-5}cm$                        

                          $=6.6\times {10}^{-5}\times {10}^{7}nm\left(\because 1cm={10}^{7}nm\right)$

                          $=660nm$