Lyman-alpha absorption
= Lyman-alpha absorption
{c}
{title2=$1s+\gamma\to2p$}
A neutral <hydrogen atom> absorbs a photon near the Lyman-alpha resonance by excitation from $1s$ to $2p$. Absorption lines in a quasar spectrum can therefore locate <hydrogen> along the sightline by their redshift, subject to peculiar-velocity and line-profile effects.