When an atom is placed in a magnetic field, each of its fine structure lines further splits into a series of equidistant lines with a spacing proportional to the magnetic field strength.

Theoretically, this is explained by recognizing that the electron has
an orbital magnetic moment

where is the angular momentum operator, is the Bohr magneton

The orbital magnetic moment gives rise to an interaction with a magnetic field proportional to , where is the magnetic field vector.

This interaction gives rise to the so called *normal Zeeman effect*.
The normal Zeeman effect would predict a number of lines equal to
, the number of eigenvalues. Note that, since must
be an integer, this number is always *odd*.

However, there is an *anomalous Zeeman effect* which shows
up particularly for atoms with odd atomic number (hydrogen, for example).
In such cases, it is found that the number of Zeeman sub-levels
is actually **even** rather than **odd**. This cannot be
explained within the normal Zeeman theory. However, it suggests
the possible existence of an angular momentum like quantity that
can take on half-integer values.