Lyman-alpha Absorption
The sharp cutoff in light seen in distant spectra caused by neutral hydrogen blocking photons at the Lyman-alpha wavelength (121.6 nm).
Lyman-alpha absorption is the specific reduction in light intensity from a distant source caused by neutral hydrogen gas absorbing photons at the Lyman-alpha wavelength (121.6 nm). This phenomenon creates the 'Lyman-alpha forest' in spectra, representing cumulative absorption along the line of sight through the universe's intergalactic medium. It is a critical tool for measuring cosmic distances and mapping the distribution of hydrogen gas.
What this means in real life
When astronomers point a telescope at a distant quasar, they see dark lines in its spectrum where hydrogen gas between Earth and the quasar has absorbed specific ultraviolet light—like looking through fog that blocks only one particular color of light.
What it isn’t
This is not the same as hydrogen emission, where hydrogen atoms release light at the same wavelength. Absorption requires pre-existing light passing through hydrogen; emission occurs when hydrogen atoms spontaneously radiate energy.
Commonly misused online
Social media sometimes conflates this with general 'light absorption' or treats it as a mysterious phenomenon unique to black holes, when it's simply a well-understood atomic physics process used routinely in astronomy.