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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

1. 10^(-17) J
2. 1 J
3. 7 × 10^(-15) J
4. 10^(-24) J

Correct answer: 10^(-17) J

Solution

The energy of the emitted wave can be calculated using the formula E = hν, where h is Planck's constant and ν is the frequency. For the 21 cm wavelength, the energy is approximately 10^(-17) J.

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