Jean Gorman, of Nayland College, asks :-

Why is lead not transparent but lead crystal glass is?

John Campbell, a condensed matter physicist at the University of Canterbury, responded.

Because they are two quite different materials. Lead, being a metal, conducts electricity through having "free" electrons, but lead glass is an electrical insulator.

Visible light is an electromagnetic wave which oscillates at about ten thousand million million times per second. Electrons are light enough in mass that they can oscillate almost in step with the light wave. We need to consider what electrons are present in the material to determine how the light wave interacts with the particular material.

In a glass, an electrical insulator, the atomic electrons are all tightly bound to atoms so they cannot move far from their equilibrium positions. This means that these electrons move, but only by a little bit and thus these orbital electrons don't make the same full motion as the light wave. The main result is that the light wave slows down to about two thirds the speed it had in air. Hence glass can be used to bend light, for example as it does in reading glasses. Not much of the light is reflected at the surface (five percent is typical), and if the glass is colourless then not much light is absorbed in the glass and overall the glass appears transparent.

In a metal, such as lead, one or two outer electrons of each atom are free to move anywhere inside the lump of metal. These conduction electrons, as distinct from the inner orbital electrons which are still bound to the atoms, can follow completely the oscillating electric field of the incident light wave. The conduction electrons oscillate with the same motion as the light wave. Because of this the conduction electrons radiate light and this manifests itself as a strongly reflected beam. Hence all electrical conductors, such as metals, are shiny, and no light is transmitted through it.

If we increase the frequency of oscillation of the light wave to be a lot faster, as in X-rays, then none of the electrons can follow the motion of the light wave so all materials, including metals, are relatively transparent to X-rays.