If the wavelength of an electromagnetic radiation is $2000°\text{A}$. What is the energy in ergs?

According to Planck’s quantum theory, $\text{\hspace{0.17em}E}=\text{h}\nu \text{\hspace{0.17em}and\hspace{0.17em}E}=\frac{\text{hc}}{\lambda }$

 Wavelength, $\lambda =2000\text{Å\hspace{0.17em}}(\text{or)}2000\times {10}^{-8}\text{cm}$

$\text{c\hspace{0.17em}\hspace{0.17em}}=\text{\hspace{0.17em}3}\times {\text{10}}^{10}\text{cm}.{\sec }^{-1};\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}Planck's constant,\hspace{0.17em}}\left(h\right)\text{ =\hspace{0.17em}6.626}\times {\text{10}}^{-27}\text{\hspace{0.17em}erg.sec}$

  $\therefore \text{E}=\frac{6.626\times {10}^{-27}\text{erg}.\sec \times 3\times {10}^{10}\text{\hspace{0.17em}cm}.{\sec }^{-1}}{2000\times {10}^{-8}\text{\hspace{0.17em}cm}}=9.94\times {10}^{-12}\text{\hspace{0.17em}ergs}$