When a source of light emitting line spectra is placed under a static magnetic field, it is observed that the spectral lines split into several components.

Zeeman in 1896 first observed the phenomenon.
Applying the magnetic field his observations were made in two directions with respect to the magnetic field directions.

1. Observations perpendicular to the magnetic field:

(a) Spectral lines split into three components
(b) Central line has the same frequency as the original line before applying the magnetic field.
(c) Central line is linearly polarized and parallel to the magnetic field.
(d) The two other components are equally separated on both sides of the central frequency.
(e) Both of them are also linearly polarized and polarization is perpendicular to the magnetic field.

2. Observations along the magnetic field direction:

(a) Central line is absent
(b) The other two components are circularly polarized

This effect is known as Normal Zeeman effect. It was also later observed that in some cases there were more than three lines. To differentiate, it was named as Anomalous Zeemen effect. In the following, we will discuss this in details.