Strange substance: Is glass truly a liquid, and how can we see through it?

Glass is an amorphous material with intriguing yet peculiar properties. But how does glass behave at the atomic level? Why can we see through it? And what exactly is glass? This is discussed by IFLScience.

Is Glass a Liquid or a Solid?

Sometimes, you might come across claims online that glass is actually a liquid rather than a solid. However, glass moves so slowly that this is imperceptible. Typically, this assertion is supported by the observation that stained glass in old churches is thicker at the bottom. This is explained by the fact that gravity has pulled the glass downward over hundreds of years. In reality, glass shares many characteristics with liquids, but it also possesses properties of solids. Therefore, glass is certainly not a liquid.

There are many types of glass, but it is easiest to focus on silicon dioxide, which is the primary component of most glasses. When silicon dioxide is heated sufficiently, it melts and becomes a liquid. However, if silicon dioxide is cooled quickly enough, it does not form organized crystalline structures like other solids; instead, it remains disordered.

According to Philip Gibbs from the University of California, Riverside, USA, as a liquid cools, its viscosity increases, which prevents crystallization. Normally, when a liquid is cooled below its freezing point, crystals form, and it solidifies. However, sometimes a liquid can become so cold that it stays in this state even below its melting point.

The scientist explains that if the viscosity of the liquid continues to rise as it cools further, it may never form crystalline structures. The viscosity keeps increasing, and eventually, the liquid transforms into an amorphous solid. While the molecules are arranged randomly, the material can still retain some solidity.

In this state, glass exhibits properties similar to those of both liquids and solids. However, glass does not flow like a liquid.

According to John Mauro from Penn State University, USA, over relatively short periods, glass behaves like a solid. But over very long periods, due to its liquid-like structure, this material undergoes a process of relaxation. This is a continuous yet extremely slow process during which the atoms in the glass gradually rearrange into a more stable structure.

The amorphous structure of glass allows sunlight to pass through it with minimal scattering. However, this does not explain why glass is transparent.

Why Can We See Through Glass?

When particles of light, or photons, hit a solid material, the material can absorb the light particles, preventing them from passing through or reflecting back. However, photons can also be reflected off solids when they are momentarily absorbed and then re-emitted.

In these instances, due to absorption, light does not pass through the material. Yet, light can traverse certain materials unchanged. If photons do not have enough energy to excite electrons to a higher energy state, the light particles are not absorbed by the material and instead pass through it.

In most materials, visible light is sufficient to excite electrons to a higher energy state, which is why they are not transparent. However, in glass, the gaps between possible energy states are large enough that visible light does not cause electrons to transition to a higher energy level.

While glass allows visible light to pass through, ultraviolet light has enough energy to elevate electrons to a new energy level. Thus, many wavelengths of light do not penetrate glass, giving it certain filtering properties against ultraviolet rays.