Wavelength of quantum of radiation is 620 nm. Number of quanta present in 10 joules is equal to $3.125\times {10}^{\mathrm{x}}.$Value of ‘x’ is equal to

$$\begin{gathered} \mathrm{Energy} \mathrm{of} \mathrm{one} \mathrm{quanta}(\mathrm{in} \mathrm{eV}/\mathrm{atom})=\frac{1240}{\mathrm{\lambda }(\mathrm{in} \mathrm{nm})}; \\ \mathrm{Energy} \mathrm{of} \mathrm{one} \mathrm{quanta}(\mathrm{in} \mathrm{ev}/\mathrm{atom})=\frac{1240}{620}=2 \mathrm{eV}/\mathrm{atom}; \\ \mathrm{Total} \mathrm{energy}\left(\mathrm{E}\right)=\mathrm{Nh\nu };(\mathrm{h\nu }= \mathrm{Energy} \mathrm{of} \mathrm{one} \mathrm{quanta}) \\ \mathrm{Number} \mathrm{of} \mathrm{quanta}\left(\mathrm{N}\right)=\frac{\mathrm{T}.\mathrm{E}}{\mathrm{h\nu }}=\frac{10\mathrm{J}}{2\times 1.602\times {10}^{-19}\mathrm{J}}=3.125\times {10}^{19} \end{gathered}$$