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If atoms are mostly empty space, why can't we see through walls?

The reason is that light interacts strongly with many materials. Even though a photon is small, it can still be absorbed by the electrons in, say, a wall. Think of atoms as a bunch of frogs (electrons) bouncing around on bungee cords attached to central points (nuclei); if a fly (electron) tries to go through a thick enough assembly like this, it's just about certain to be eaten by a frog. Even if there's a lot of space between the frogs, sooner or later, a frog will get the fly.

Now, obviously there are some problems with this analogy (it doesn't explain how light can pass through glass, for example), but I think you can see what is going on. If you make the wall very thin, some light will pass through it.

Richard Barrans Jr., Ph.D.

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