 Vector magnetographs measure the longitudinal (line-of-sight) component of the magnetic field separately from the transverse (image-plane) components, using different aspects of the Zeeman splitting that affects the wavelength of emission and/or absorption spectral lines in the presence of a magnetic field.The Zeeman splitting is caused by the fact that individual atoms are magnetized due to the circulating motion of electrons bound to them.Emission or absorption of a photon changes the magnetic moment of the atom.