You searched for:
“twenty one centimeter radiation”
The common name given to radio waves emitted at 1,420 megahertz as the result of the "flipping over" of the electron in a hydrogen atom in order to oppose the spin direction of the central proton: The twenty-one centimeter radiation is an example of line emission.
This entry is located in the following unit:
Astronomy and related astronomical terms
(page 27)