Only a hot, thin gas can produce an emission line.

Question

Quizplus · Accepted Answer

An emission line is created when electrons within an atom are excited to higher energy levels and then return to lower energy levels, emitting a photon in the process. This can only occur in a hot, thin gas where there is enough energy to excite the electrons and little enough density for the emitted photons to escape without being absorbed by other particles. An emission line is produced when the electrons in the gas atoms are excited to a higher energy level and then return to their ground state, emitting a photon of a specific wavelength. This process typically occurs in hot, thin gas clouds, such as those found in nebulae, where there is enough energy to excite the electrons and the gas is not dense enough to cause significant absorption or scattering of the emitted photons.

Only a Hot, Thin Gas Can Produce an Emission Line