Absorption and emission
When The smallest part of an element that can exist. absorb energy, through heating, from electricity, or by absorbing Energy travelling as waves in the form of changing electrical and magnetic fields., the electrons at a particular level can be pushed up to higher levels (at bigger distances from the nucleus). In time, they jump back down to a lower level radiating energy in the form of A transverse wave caused by oscillations in an electromagnetic field..
The frequency of the radiation depends on the size of the jump that the electron makes. Sodium has Specific amounts of energy that electrons have when they orbit a nucleus is a particular shell. Electrons that gain energy may move to a higher energy level (a shell further from the nucleus). Electrons that lose energy may move to a lower energy level (a shell closer to the nucleus). that cause the electrons to emit yellow light as they drop from one to the other. The energy levels in mercury are further apart and electrons emit Electromagnetic radiation with a frequency between that of visible light and X-rays. when they jump back down.
In the diagram, the colours of the 'down arrows' represent the colour of the light emitted as the electron makes that drop.