Question

What is line emission spectrum?

Answer 1

It is a sequence of lines of different color/frequency that represents a kind of "photograph" of the structure of an atom.

Explanation:

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=hf`). 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).

As a real example you have Hydrogen that in emission shows colors corresponding to various transitions: