Abstract
An apparatus is described which enables the user to measure continuous refractive index spectra of opaque and transparent optically flat samples. The method of measurement is based on finding the Brewster's angle beta(B) of the sample. By modulating the angle of light incidence, the output signal of a lock-in amplifier, tuned to the modulation frequency, changes sign abruptly at beta(B). The lock-in signal is applied to a dc motor which drives a goniometer in that way that the sample is continually held at beta(B). Provided that the sample extinction coefficient k < or approximately 0.5, the accuracy of the refractive index measurements is better than 1%, or even 0.1% where k is approximately equal to 0.
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