# What is line emission spectrum?

In a very simplistic way you can consider a gas of atoms of a substance illuminated by light. This means that the gas is receiving energy carried by the photons of light (as in $E = h f$). The atoms of the gas absorb energy so that their electrons undergo transitions to higher allowed orbits inside the atom (specific for that particular atom/element).
After a short while the atoms emit the surplus energy (Emission) and we can "see" this emission as photons of light of frequency $f$ related to the orbital(s) distance(s) or separation(s).