Question

Why splitting of spectral lines takes place when the source giving the spectrum is placed in a magnetic field?

Answer 1

When the source giving the spectrum is placed in a magnetic field atomic level splitting takes place which is known as Zeeman Effect.

Due to the interaction of degenerate electrons with magnetic field displacement from degeneracy occurs.  This results in extra lines appearing. As the angular momentum state has associated magnetic dipole moment, magnetic field separates these states into different energy levels-

i) one state elevating the energy

ii) another lowering the energy and

iii) one remaining at the same energy level.

Hence in addition to original lines we find more lines.