Hi, I am not a physicist. I am trying to understand something about the behaviour of materials for computer graphics applications. I am trying to get an intuitive understanding of refraction. As far as I understand it - and please correct me if I am wrong - lightwave's phase velocity is altered, usually causing a change in direction when it enters another medium. What causes this? Most of the descriptions I have read so far are circular, in that the direction change is defined as being a result of snell's law, or a result of the refractive index, etc. But this the same phenomenon in different forms. What is actually happening that causes the phase velocity change?