 In a magnetic field, photons emitted with different polarizations gain or lose energy depending on their orientation relative to the surrounding magnetic field, changing the characteristics of the spectral line—some polarization components are blue-shifted or red-shifted relative to the line's reference wavelength, by a factor proportional to the field intensity.Specifically, the circular-polarized component of the light is shifted in wavelength proportional to the field strength in the direction of the observer, and the wavelength shift of the vertical and horizontal linearly-polarized components measures the field strength in those directions.A vector magnetograph works in a very narrow waveband around a single spectral line, for example the 525.02 nm 'Fe I' line from neutral (non-ionized) iron.