A few things you should know first...
-A particular type of atom absorbs
only certain wavelengths of light corresponding to the allowed energy levels
of its electrons. When observing the spectrum of light that has passed
through say a cloud of gas, the absorbed light does not make it and a dark
band, or spectral line, appears at that wavelength.
-Electromagnetic radiation, or light,
is made up of an oscillating electric and magnetic field. The direction
of oscillation of the electric field defines the lights polarization, which
may be along a line or in some elliptical or circular motion.
The Zeeman Effect
In the presence of a strong magnetic field, a spectral line
normally at one wavelength splits into two or three components of circular
or plane polarization
The amount of splitting corresponds to the strength of the
magnetic field
The polarization gives the direction of the magnetic field
The Sun's Magnetic Field
Normal sunlight has no preferred direction (polarization)
When looking at an active region (which always has a strong
magnetic field), we observe the Zeeman Effect
With no magnetic field, an electron can jump to only one
distinct energy level. Therefore, it can only absorb one distinct wavelength
of light and creates a narrow line in the spectrum. In the presence of
a magnetic field, an electron can make a transition to three slightly different
energy levels. This allows the electron to absorb 3 different wavelengths
of light creating a broadened or totally split line in the spectrum.
So the direction, distribution, and strength of the magnetic
field on the photosphere can be determined!
