The Zeeman Effect

In the presence of magnetic fields, spectral lines are split. This is called the Zeeman effect. Electrons in their orbits in atoms have an angular momentum associated with their motion called the orbital angular momentum (angular momentum is discussed in Chapter 7 on the formation of the Solar System). This angular momentum is a vector quantity, because it has a magnitude and a direction associated with it. The Zeeman effect can be interpreted physically in terms of the precession of the orbital angular momentum vector in the magnetic field, similar to the precession of the axis of a spinning top in a gravitational field. The pattern of spectral line splitting is a a signature that a magnetic field is present, and the magnitude of the splitting is a measure of the strength of the field.

Zeeman Splitting

The Zeeman splitting of spectral lines is associated with what is called the orbital angular momentum quantum number L of the atomic level, which measures the magnitude or the orbital angular momentum. This quantum number can take nonnegative integer values. The number of split levels in the magnetic field is in general equal to 2L + 1. The box below gives a more detailed explanation of Zeeman splitting and here is an animation illustrating the Zeeman effect.

Polarization and Zeeman Splitting

The lines corresponding to Zeeman splitting also exhibit polarization effects. Polarization has to do with the direction in which the electromagnetic fields associated with a light wave are vibrating. This in turn, can have an effect on whether the spectral light can be observed. One practical example in astronomy of such polarization effects is that in the top right diagram the middle transition is polarized such that it cannot be observed from directly over a surface perpendicular to the magnetic field. As a consequence, usually when looking directly down on sunspots (which have strong magnetic fields) only two of the three transitions shown can be seen. We shall discuss sunspots in more detail in Chapter 17.

Technically Speaking: Zeeman Splitting of Atomic Levels

Atomic physicists use the abbreviation "s" for a level with orbital angular momentum quantum number L=0, "p" for L=1, "d" for L=2, and "f" for L=3. It is also common to precede this designation with the integer principle quantum number n. Thus, "2p" denotes a level that has quantum numbers n=2 and L=1. In the example shown above the lowest level is an "s" level, so it has L=0 and 2L + 1 = 1, so it isn't split in the magnetic field, while the first excited state has L=1 ("p" level), so it is split into 2L + 1 = 3 levels by the magnetic field. Thus, the possible transitions between these two levels are split into three transitions. If both the initial and final levels have L not equal to zero, both will be split into more than one level and an even more complicated set of splittings for the spectral line is possible.