Hi, I am reviewing some material on refraction/Huygens' principle and I would like some help with a couple of technicalities. 1) Why does light slow down when it enters a denser material? (i.e details on scattering/ microscopic effects )Are the resultant wavefronts closer together? 2) Does it make sense to talk of the acceleration or deceleration of light? Does light decelerate when it enters such a denser material?