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The Engineer: Why glass is invisible

It’s the size of the subatomic band gap in its electron shell. Obviously.

April 10, 2024  By Claudio Sacilotto

As a young child, I used to wonder what made glass invisible. Was it magic? Or just something that only scientists understood. I finally decided to take a trip down nerd road and figure out what makes this stuff so completely transparent. After all, this is the principle material in our industry. 

The main raw material for glass is sand. Typically, sand is mostly silica in the form of quartz, which in its pure form is transparent, clear and colourless. Impurities in the sand will impede the glass’s colourlessness and transparency, therefore sand needs to be purified so that it can be made invisible. When molten glass is quickly cooled, the glass molecules (mostly silica, soda ash, and lime) cool into an amorphous solid without the crystalline structure that is common in many other solids. There are no grain boundaries or other crystallographic defects that could scatter light. As it is losing energy, the glass matrix (in an attempt to minimize its surface energy) will form an ultra-smooth surface that is essential for transparency, as there are no angles to refract or reflect light in various directions.

But how does light pass completely through? Light energy comes in the form of photons. When photons strike a material, they can either pass through it, get absorbed or are reflected. When we look into the subatomic structure of the atoms that make up glass molecules, we perceive electrons “buzzing” around a dense nucleus. The reality of these “buzzing” electrons is that they are fixed to prescribed orbitals or electron energy states. The gap between the various electron energy states is known as the band gap. Every material has its characteristic band gap. It’s these gaps that are responsible for many of its material properties. It just happens that the band gap for glass is fairly large. Metals typically have small or no band gaps and this is the reason why they are opaque. It’s no coincidence that materials with very small band gaps are also electrically conducting as small band gap differences are also responsible for electrical conduction.

The band gap of glass is such that photon energy from visible light (in the 400 to 700 nanometer range) is not high enough to promote its electrons across the band gap. Therefore, photons pass right through the material unobstructed. For other materials such as metals, the energy band gap is small. Therefore, visible light photon energy will get absorbed by the electron in the metal and be promoted across the band gap. The photon stops there. This is why materials such as metal are opaque. Conversely, higher energy radiation from the electromagnetic spectrum, such as ultraviolet light (in the 10 to 330 nanometer range) has sufficient energy to promote the electrons in glass across the band gap. Just like a metal, the photons lose their energy to promote the electrons and the photons stop in their course. For the most part, glass is opaque to most UV light (with the exception of UV-A light in the range of 330-400 nanometers). Needless to say, it is difficult to suntan behind a window. 


So, you may also wonder why sand, the raw material for glass, is not transparent. This has to do with the fact that each grain of sand is a small crystalline glass cube. These cubes have facets that face many directions. The refraction of light with the facet will scatter it in many ways and the overall collection of these cubes of quartz results in an opaque appearance. Wetting sand further increases the refractive index differences of the sand with water, so the sand will appear to be darker. Interestingly, immersing quartz sand in a liquid with the same refractive index will result in the sand becoming invisible as there will be no more light scattering.

The invisibility of glass is a result of its unique atomic structure and light transmission properties. Whether in architecture, art, or the lenses of my glasses, the invisible wonder of glass continues to captivate and enrich our world. •

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