Abstract
The equations describing the penetration of resonance gamma -rays through a crystal subject to ultrasonic vibrations are derived for the case when refraction effects must be taken into account. It is shown that the formation of the observed Mossbauer spectrum is determined in this case: firstly, by a certain effective refractive index dependent on the ultrasonic amplitude and frequency and, secondly, by the forced Mandelshtam-Brillouin effect.