1212. Why can we see through glass and some minerals? — KH, Newport Beach, CA
Light consists of electromagnetic waves—fluctuating electric and magnetic fields that travel through space at enormous speeds. As light passes through an insulator such as glass, diamond, quartz, or salt, the light's fluctuating electric and magnetic fields cause electric charges in the insulator to vibrate back and forth. This interaction between light and the charged particles in a material is the first step in absorption—the material is "trying" to absorb the light. But light carries energy with it and any material that absorbs light must be prepared to accept the light's energy. The charged particles in insulators generally have no quantum states that allow them to accept that light energy. As a result, the insulator's charged particles respond to the light as it passes, but they can't actually absorb the light. The light simply passes through the insulator. However, the light is delayed by its interaction with the charged particles in the insulator (the speed of light in a material is less than the speed of light in vacuum) and the light may be redirected (reflected or scattered) by encounters with inhomogeneities. So glass and the other insulators don't absorb light and are often transparent. Those that aren't transparent are usually white—they scatter light in all directions.