To answer this question, we first need to understand why are things opaque or in other

words, why do the things around us absorb visible light?

When a photon (a particle of light) hits an object it has 3 options:

Either it will get absorbed, or it will get reflected or it will pass right through it.

When light gets absorbed in a material what happens is that the light energizes the electrons

of the atoms that make up that material. They collide and the electron receives just enough

energy to jump to the higher energy level.

But electrons can't just jump everywhere they want. They can't jump in between these energy

levels. They are either on this one or this one... they can't be halfly-energized.

And this is the reason that the glass is transparent. In glass, these energy levels are so far apart

that the visible light doesn't have enough energy to push that electron to the higher

energy level. And since it can't do that, and since electrons can't be halfly-energized,

the light goes right through the glass unchanged.

On the other hand, UV light has enough energy to do just that. That's why glass is as opaque

to UV light as wood is to the visible light.