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Maxwell2D: Michelson interferometer, unequal path lengths

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

Electric field animation

Time-averaged Poynting vector

This is an animation of the component of the electric field out of the plane of the page.The direction in which energy is travelling at any instant is proportional to the cross product of the electric and magnetic fields, known as the Poynting vector. The arrows show the direction of the Poynting vector averaged over the simulation, while the colours show its strength.