Daniel is tapping on a window and wondering why he can see straight through it.

The wall next to it is solid. The glass is solid. So why does one block light and the other doesn't?

His first guess is thickness. Thicker wall, less light gets through. Reasonable. Wrong.

You can have glass ten centimeters thick and see straight through it. You can have paper thinner than your fingernail that blocks light completely. Thickness has nothing to do with it. The answer goes all the way down to what's happening inside the material at the atomic level — and it comes down to one thing: whether the electrons can absorb the light.

Here's how it works. Light is made of tiny packets of energy called photons. When a photon hits a material, one of three things happens — it gets absorbed, it gets reflected, or it passes straight through. Wha ...