"Glass is like water in some respects. The atoms of glass are silicon, oxygen, calcium, and sodium, all small atoms. The electrons in small atoms are close to the nuclei. The forces restoring the electrons to their points of zero force are strong. The electrons resonate at frequencies in the ultraviolet range. Photons of visible light are transmitted, rather than absorbed. Some of the photons are scattered, so that very thick glass is noticeably less than perfectly transparent."