Question

An important spectral emission line has a wavelength of 21 cm. The corresponding photon energy is
$(h=6.62\times {10}^{-34}Js$; $c=3\times {10}^{8}m/s)$

• A. $5.9\times {10}^{-4}eV$
• B. $5.9\times {10}^{-6}eV$
• C. $5.9\times {10}^{-8}eV$
• D. $11.8\times {10}^{-6}eV$

Answer 1

The correct option is B

$5.9\times {10}^{-6}eV$

$E=\frac{hc}{\lambda }=\frac{3\times {10}^8\times 6.62\times {10}^{-34}}{0.21\times 1.6\times {10}^{-19}}=5.9\times {10}^{-6}eV$