Question

The 21 cm radio wave emitted by hydrogen in interstellar space is due to the interaction called the hyperfine interaction is atomic hydrogen. The energy of the emitted wave is nearly

• A. ${10}^{-17}$ Joule
• B. 1 Joule
• C. $7\times {10}^{-8}$ Joule
• D. ${10}^{-24}$ Joule

Answer 1

The correct option is D ${10}^{-24}$ Joule

$E=\frac{hc}{\lambda }=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{21\times {10}^{-2}}=0.94\times {10}^{-24}\approx {10}^{-24}$ J