Why Is Glass Transparent?

Have you ever walked into a glass door because you didn't see it? Or marveled at how a greenhouse traps warmth while letting light flood in? Glass performs a kind of magic trick right before our eyes—being simultaneously solid yet allowing us to see straight through it as if nothing were there at all. How can something be physically present yet visually absent? Why is glass transparent? It seems like a simple question with an obvious answer: light passes through glass because glass is transparent. But that's circular reasoning. The real question is: why does light pass through some solids but not others? Most solids block light completely. A piece of wood, metal, or stone in your path means you can't see what's behind it. But place a window in your path, and light streams through as if nothing's there. This is deeply strange when you think about it. Let's address what might seem like the obvious explanation—our first potential answer that ultimately won't satisfy us.