Optical measurements on silicon and germanium together with Hall measurements on germanium were performed on crystals containing predominantly edge or screw dislocation arrays. An apparent extended absorption tail in deformed germanium and silicon was found to depend markedly upon the direction of light propagation relative to the active slip planes. Photographs of the transmitted radiation through deformed silicon showed that the apparent extended absorption tail was due to strong anisotropic scattering from active slip planes. Hall measurements on sections from plastically deformed germainium are interpreted to give evidence of dislocation acceptors located in the lower half of the band gap.