An Interferometer for Direct Recording of Refractive Index Distributions

The well established principle of division of amplitude in two-beam interference is used to locate constant optical thicknesses in an experiment where the refractive index is a continuously varying function of position. This is applied to the experimental situation of determining the temperature distribution around a plane heat source in a transparent liquid. It turns out that it is most convenient to study the interference in the light reflected by the front and rear windows of the cell. The highly coherent light from a helium-neon gas laser is a basic requirement for this interferometry, where one is using optical path differences of the order of half a meter. The focusing of the optical system is discussed in some detail. A second order aberration theory gives the condition for the elimination of what corresponds to the Wiener skewness.