question_answer

The 21 cm radio wave emitted by hydrogen in interstellar space is due to the interaction called the hyperfine interaction in atomic hydrogen. The energy of the emitted wave is nearly:                                                                        [AIPMT 1998]

A)        \[{{10}^{-17}}\,J\]                 

B)  1 J                  

C)  \[7\times {{10}^{-6}}\,J\]

D)                 

\[{{10}^{-24}}\,J\] Correct Answer: D Solution :

The energy of emitted photon

\[E=\frac{hc}{\lambda }\]

Given :  \[\lambda =21\,cm=0.21\,cm\]

\[\therefore \]      \[E=\frac{6.6\times {{10}^{-34}}\times 3\times {{10}^{8}}}{0.21}={{10}^{-24}}\,J\]