We investigate the propagation of light for metrics in which the approximation of geometrical (ray) optics breaks down due to nonadiabatically rapid (in space or time) variations of the curved-spacetime geometry. This may lead to wavelength-dependent effects such as diffraction and dispersion. The rapidly-expanding cosmological spacetimes are good candidate metrics for such effects. We find that conformal invariance prevents such violations of adiabaticity for the spatially-flat Friedmann Robertson-Walker metric; but if one breaks conformal invariance via the anisotropic (but homogeneous) Kasner metrics, then the propagation of light waves will be shown to deviate from the predictions of simple ray-tracing. ©2001 American Institute of Physics.