Why does radiation of the visible spectrum move through silicon, water, ice, glass and so on? I've been wondering about this for a long time, and have been looking at various texts on the net, and the most common answer found is that it a) is because the atoms in the compound are unordered, and/or b) because the electrons in for example glass transmit the photons passing through it. Now, none of the texts give a very good explanation to as what factor(s) are important when determining whether a compound will be transparent to a certain wavelength. As far as I've understood, the wavelength of the radiation passing through has a lot to say. Visible light goes through glass, but UV and IR do so poorly. Why? Is it because of a relation between the density of glass and the wavelength of light? Is it the number of valence electrons in each atom? Or is it the type of bonds between the atoms? Or even between the molecules? For instance, I can't find anywhere whether UV goes through silicon. Or better, whether x- or gamma-rays would go through a solid iron door. I've found this here at physicsforums, but that discussion doesn't give the fundamental reasons for whether radiation would go through something. Any comments at all are appreciated!