A change of direction and possibly amplitude of an electromagnetic, acoustic, or any other wave propagating in a material medium, homogeneous on the scale of the wavelength, as a consequence of spatial variation in the properties of the medium. This variation can be abrupt (on the scale of the wavelength), as in refraction of electromagnetic waves because of an air–water interface, or gradual, as in refraction of electromagnetic waves by the atmosphere. Refraction is distinguished from (specular) reflection in that the direction of propagation of the refracted wave does not have a component opposite the direction of the incident wave. See atmospheric refraction, refractive index; Compare diffraction, scattering.