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A hydrogen atom with proton and electron spins aligned (top) undergoes a flip of the electron spin, resulting in emission of a photon with a 21 cm wavelength (bottom) The hydrogen line, 21 centimeter line, or H I line [a] is a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms.
The waterhole, or water hole, is an especially quiet band of the electromagnetic spectrum between 1420 and 1662 megahertz, corresponding to wavelengths of 18–21 centimeters. It is a popular observing frequency used by radio telescopes in radio astronomy .
For example, the 2 → 1 line is called "Lyman-alpha" (Ly-α), while the 7 → 3 line is called "Paschen-delta" (Pa-δ). Energy level diagram of electrons in hydrogen atom. There are emission lines from hydrogen that fall outside of these series, such as the 21 cm line.
Detecting the 21 cm emission from this time, all the way through to the end of reionization, has been proposed as a powerful way of studying early structure formation. [9] This period of the Universe's history corresponds to redshifts of z ≈ 30 {\displaystyle z\approx 30} to z ≈ 6 − 12 {\displaystyle z\approx 6-12} , implying a frequency ...
A spectral line may be observed either as an emission line or an absorption line. Which type of line is observed depends on the type of material and its temperature relative to another emission source. An absorption line is produced when photons from a hot, broad spectrum source pass through a cooler material.
The amount of energy needed to reverse the spin of the electron is equivalent to a photon at the frequency of 1.420 405 751 768 GHz, [1] which corresponds to the 21 cm line in the hydrogen spectrum. Hydrogen masers are very complex devices and sell for as much as US$235,000. [2] There are two types to be distinguished: active and passive.
The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space.
10 cm = 1.0 dm – wavelength of the highest UHF radio frequency, 3 GHz; 12 cm = 1.2 dm – wavelength of the 2.45 GHz ISM radio band; 21 cm = 2.1 dm – wavelength of the 1.4 GHz hydrogen emission line, a hyperfine transition of the hydrogen atom; 100 cm = 10 dm – wavelength of the lowest UHF radio frequency, 300 MHz