The difference between glass and wood

Let's take the example of windows. A window is usually made up of a glass pane set in a wooden framework. It makes a home feel bright, warm and welcoming because they let light enter. But why should a glass window be any more transparent than the wood that surrounds it? After all, both materials are solid, and both keep out rain, snow and wind. Yet wood is opaque and blocks light completely, while glass is transparent and lets sunshine stream through unrestricted.

You may have heard some people - even some science textbooks - try to explain this by saying that wood is a true solid and that glass is a highly viscous liquid. They then go on to argue that the atoms in glass are spread farther apart and that these gaps let light squeeze through.

But really, what is glass?

In reality, glass isn't a liquid at all. It's a special kind of solid known as an amorphous solid. This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat, orderly crystals, they arrange themselves randomly. As a result, glasses are mechanically rigid like solids, yet have the disordered arrangement of molecules like liquids. Amorphous solids form when a solid substance is melted at high temperatures and then cooled rapidly - a process known as quenching.

What happens when light hits glass

Light is made up of particles called photons. When they hit 'stuff', three things occur - reflection, absorption, and transmission.

When light hits glass, only about 4% of the photons will be reflected.

Some of the photons that are not reflected will be absorbed. That means the photons will interact with the atoms and molecules of the material that is impacted. Its energy will serve to heat up the material.

Most of the photons are transmitted. That is, if the atoms and molecules of the material do not have such "energy transitions" that match the energy of the photon, then the photon will be absorbed and then re-emitted by the atoms/molecules.

The important difference between glass and opaque materials like wood is that glass happens to have energy transitions that match the energy of visible photons. So photons are absorbed, and then reemitted by each of the atoms it hits. So essentially most of the light that falls on glass is simply re-emitted and it is this unique property that makes glass transparent.

However, glass does absorb UV photons, which is why you can't get a suntan through glass.