How does an atomic spectrum get affected by external electromagnetic fields?
A spectrum contains spectral lines pertaining to each transition an electron can make from one energy level to another.
An example is atomic spectra:
Electronic transitions occur when an electron is excited into a new energy level, or when an excited electron relaxes into a lower energy level.
However, when you apply an electric or magnetic field, you may cause the energy levels to split, introducing more possible electronic transitions than before.
One case (probably the one you're looking for) is the splitting of degenerate energy levels using an electric field---the Stark Effect---like this:
Focus on only how the energy levels go from one to multiple levels; that's the important part here. Ignore the S, P, and D symbols.
This would cause the splitting of spectral lines in accordance with the electric field, turning one line into multiple lines:
The Zeeman effect is similar, but it is accomplished using magnetic fields instead.
This, again, yields a splitting of spectral lines in accordance with the magnetic field: