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The wavelength 21 cm emitted by atomic hydrogen in interstellar space belongs to

1. Radio waves
2. Infrared waves
3. Microwaves
4. $\gamma$-rays

Correct answer: Radio waves

Solution

A wavelength of 21 cm lies in the centimetre range, which is classified as radio waves in the standard electromagnetic spectrum used in school-level physics. It is not in the infrared or gamma region, and microwaves are typically shorter centimetre-to-millimetre wavelengths. Hence the correct choice is radio waves.

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