Measurement of the Refractive Index of Lucite by Recoilless Resonance Absorption

A method of frequency-modulating a monochromatic electromagnetic wave by varying the optical path length between the source and detector is described. The method has been applied to the measurement of the refractive index of Lucite for the 0.86 A radiation emitted from ${\mathrm{Co}}^{57}$; the small frequency shift was detected by recoilless resonance absorption. The refractive index was found to be $1-n=(1.29\pm 0.03)\times {10}^{-6\mathrm{}}$, in agreement with classical theory.