Interferometric measurement of refractive index

In this paper a simple method is described for measuring the principal refractive indices of crystals using a Michelson interferometer and a single principal section of the material. This work formed the basis of the final year BSc project in laser physics and optoelectronics for one of the authors. The refractive indices of a sapphire crystal obtained by this technique are accurate to the third decimal place. The procedure is then used to obtain the three principal refractive indices of a new biaxial crystal, potassium niobium borate, whose refractive indices were previously unknown.