Why is glass transparent? | April 3, 2015
I’ve always wondered why glass was transparent to visible light when most other materials are opaque. I finally found a (somewhat wordy) explanation in: Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World
So why is it that glass has this apparently miraculous property of transparency? How is it that light can travel through this solid material at all, while most other materials will not allow it? After all, glass contains all of the same atoms that make up a handful of sand. Why in the form of sand should they be opaque and in the form of glass transparent and able to bend light?
Glass is made of silicon and oxygen atoms, as well as a few other sorts. Within every atom there is a central nucleus, which contains protons and neutrons, surrounded by varying numbers of electrons. The size of the nucleus and the individual electrons is tiny compared to the overall size of the atom. If an atom were the size of an athletics stadium, the nucleus would be the size of a pea at its center, and the electrons would be the size of grains of sand in the surrounding stands. So within all atoms—and indeed all matter—there is a majority of empty space. This suggests that there should be plenty of room for light to travel through an atom without bumping into either an electron or the nucleus. Which indeed there is. So the real question is not “Why is glass transparent?,” but “Why aren’t all materials transparent?”
Inside an atomic stadium, to continue the analogy, the electrons are only allowed to inhabit certain parts of the stands. It is as if most of the seats have been removed and there are only certain rows of seats left, with each electron restricted to its allotted row. If an electron wants to upgrade to a better row, it has to pay more—the currency being energy. When light passes through an atom it provides a burst of energy, and if the amount of energy provided is enough, an electron will use that energy to move into a better seat. In doing so, it absorbs the light, preventing it from passing through the material.
But there is a catch. The energy of the light has to match exactly that required for the electron to move from its seat to a seat in the available row. If it’s too small, or to put it another way, if there are no seats available in the row above (i.e., the energy required to get to them is too large), then the electron cannot upgrade and the light will not be absorbed. This idea of electrons not being able to move between rows (or energy states, as they are called) unless the energy exactly matches is the theory that governs the atomic world, called quantum mechanics. The gaps between rows correspond to specific quantities of energy, or quanta. The way these quanta are arranged in glass is such that moving to a free row requires much more energy than is available in visible light. Consequently, visible light does not have enough energy to allow the electrons to upgrade their seats and has no choice but to pass straight through the atoms. This is why glass is transparent.