### Dielectric constant distribution
| A *Fourier transform spectrometer* consists simply of a *Michelson interferometer* but with with one of the two mirrors movable. When one of the path lengths is changed by a quarter of a wavelength (as in this example), the radiation from the two paths destructively interferes in the rightward direction. Instead, the light constructively interferes in the downward direction (unlike in the previous case). The frequency spectrum of incoming light may be obtained by measuring the intensity of the outgoing radiation to the right as a function of the path length, then performing a Fourier Transform on it. This principle is used by a number of meteorological satellites instruments, such as **AIRS** and **IASI**. *Wikipedia link: Fourier transform spectroscopy* |