Rectilinear propagation

Rectilinear propagation is a wave property which states that waves (move or spread out) in straight lines. This property applies to both transverse and longitudinal waves as well as electromagnetic waves (light). Even though a wave front may be bent (e.g., 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 refractive index of the material.