Number 27 (Story #4), March 28, 1991 by Phillip F. Schewe and Ben Stein THE SPEED OF LIGHT CAN BE REDUCED BY A FACTOR OF 10 in certain disordered materials, according to Ad Lagendijk of the University of Amsterdam. The zigzag scattering of light in opaque substances like milk or white paint is a common phenomenon, but Lagendijk believes that if the size of the obstacles (in this case individual milk globules or pigment droplets in paint) is comparable to the light's wavelength then the light waves may be trapped for a time in the droplets, as if in a cavity, before moving on to another scattering encounter. If this retardation is not taken into account, Lagendijk insists, experimental determinations of the light's mean free path--the average distance between scattering with an obstacle--will be incorrect. (Science News, 23 March 1991.)