Rectilinear propagation

Rectilinear propagation is a wave property which states that waves propagate (move or spread out) in straight lines. This property applies to both transverse and longitudinal waves. Even though a wave front may be bent (the waves created by a rock hitting a pond) the individual waves are moving in straight lines. In the sense of the scattering of waves by an inhomogeneous medium, this situation corresponds to the case $n = 1$ , where $n$ is the index of the material.