A simple method is used to calculate the contribution of the rapid rise in optical absorption near the absorption edge of semiconductors to the index of refraction near the edge. Other contributions to the index are also estimated. The results give qualitative agreement with the results obtained for PbS by Riedl and Schoolar, and very good agreement with Marple's prism measurements of the index for GaAs. Some applications to injection lasers are discussed. The temperature and pressure dependence of the index of refraction near the absorption edge is estimated, and agrees well with experimental results for GaAs. © 1964 The American Physical Society.