What were the absorption clouds doing in the PRS questions?
An absorption cloud is just a cloud of cool gas. Gas is made out of atoms. Electrons in atomic matter can absorb photons, but only at certain specific frequencies. So light with a continuous spectrum passing through a cloud of gas will have only certain frequencies absorbed from it. That’s what makes an absorption line: photons of some particular frequencies get eaten up by the gas. Any cool gas will do, and each type of gas will have a characteristic set of lines.
