This one has also puzzled me for quite some time. How come that glass, water and diamonds for instance are transparent while for example graphite is not? Furthermore, since the light travels at lower speed in glass and in water than it does in vacuum, it must mean that the photons interact with the material (colloquially referred to as "bouncing" on the electron as what I've heard). However, if the photons interact with the material, how can they preserve the angle with which they entered the glass, water or whatever it might be? Why isn't the picture just blurred out? I'd like to get rid of this ignorance of mine.