Spectral line emission involves the transition of electrons in atoms from a higher energy level to lower energy level. When this happens, a photon is emitted with the same energy as the energy difference between the two levels. The emission of this photon at a certain discrete energy shows up as a discrete 'line' or wavelength in the electromagnetic spectrum. The 21-cm spectral line of neutral hydrogen, as mentioned earlier, is extensively used by radio astronomers. Similarly, the OH maser lines at 1612, 1665, 1667 and 1720 MHz.