GLASS - or how does the opaque get transparent · Glass is now a matter of course, but few know how and why glass became what it is. See the link below. Here we focus on the use of glass as a carrier and spreader of light, either in its original form as clear or transparent glass, whereby the light is distributed more or less unfiltered into the room, or as an opal glass, which illuminates the light on its whole surface and then gently and atmospherically spreads it to its environment. Since the characteristic of an incandescent lamp can correspond with a complete luminaire, we also list the most important examples here, matching fixtures to the incandescent lamps can also be found elsewhere.
more about glass: https://en.wikipedia.org/wiki/Glass