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The Transparent World of Glass

The world renowned Louvre built from glass

It may be hard to believe, but glass, that invisible and fragile material, is considered to be one of the most important materials used to shape the world of today. Whether it be holding your morning ice latte or encasing those sky piercing structures in town, glass has fundamentally built the modern world we live in. In fact, our understanding of the core sciences would not be possible without the glass in our trusty microscopes and glassware.

So, yes: glass is essential to us. But why is that exactly?

Transparency of Glass:

Well, obviously, one of the intrinsic properties of glass is that it is transparent. Glass allows light and other forms of electromagnetic waves to pass through while blocking out most other matter. To understand why glass is transparent, we first need to understand what happens when light interacts with matter. When light is incident on matter, photons, the tiny packets of energy within light, will interact with the electrons of the atoms comprising the matter, exciting the electrons. They will then move them up to higher energy levels within the atom and eventually emit a photon once the electron returns to its original energy level. However, the interaction will only take place if the photon has the specific amount of energy corresponding to the energy differences between the levels, and the photon being emitted will have that same energy. This is the key to transparency. Materials that are transparent have energy levels that do not correspond to visible light, but do to other electromagnetic waves - and thus let light photons pass by.

Comparison between crystalline (right) and amorphous (left) Silica

But why specifically is glass transparent? Well, the property derives from the peculiar material structure of glass. Glass at its simplest form are oxides such as silica (SiO2), which naturally have a regular arrangement of atoms in a lattice known as a crystalline structure. Crystalline structures are known to be opaque and the oxides of glass are no different. But what differs the oxides in glass and in their crystalline forms is that glass has an amorphous structure. You could imagine the structure as if the atoms of a liquid, which are randomly arranged, were fixed in place. With this type of structure, the electrons only absorb and emit ultraviolet light, allowing visible light through, hence the transparency as mentioned prior. This also coincidentally explains how glass protects us from ultraviolet light, since it absorbs and reflects most of it.

Production of Glass

There are many methods to produce glass, such as furnace glass blowing and splat quenching. These methods all share the same principle of converting crystalline substances to their amorphous state. The substances must first be heated to extremely high temperatures to reach molten state. For instance, take silica glass, the most common form of the material: silica must be heated to 1710˚C in order to work with it. In molten state, the molecules are mobile but still compact, allowing it to weaken the forces keeping them in their crystalline form. It is then moulded into shape and cooled to a solid.

Glass blowing is a very popular form of glass production

The cooling process is where the secret of glass lies. If left to cool normally, the atoms would just slowly return to their crystalline arrangement as they are not losing energy quickly enough, resulting in an opaque solid. But if cooled rapidly at around 2000˚Cs-1, the atoms will lose energy more rapidly than they can return to its orderly arrangement, thus resulting in the ‘frozen liquid’ like amorphous structure once it reaches a certain temperature. The temperature where this change occurs is known as the glass transition temperature.

Shortcomings of Glass

Of course, as great as glass is, it has its fair share of downsides. Phrases like “as fragile as glass” derive from the tendency for glass to shatter if relatively minimal force is applied. There is high internal tension within glass due to the rapid cooling freezing the molecules in places where they have not reached equilibrium, resulting in the dramatic and explosive shattering once released. Hence glass is considered a hazard once broken, made worse with the razor sharp shards left behind.

Glass shattering is very common

Additionally, despite glass being relatively easy to recycle in theory, people generally tend not to do so. Due to the excessive energy required to melt down the glass and rapidly cool it, as well as the extreme hassle to sort the different types of glass, recycling is typically considered impractical. Also, similar to plastics, glass can take centuries to decompose due to its lack of biodegradability, becoming a hazard to all for several lifetimes.

The Future

Glass has been a part of life for centuries, yet to this day, it is continuously evolving with the improvements in technology and research. Extraordinarily innovative types of glass have been synthesised, such as Aluminium Oxynitride (ALON), which is stronger and harder than diamond. Innovators have also integrated various technologies with glass, such as smart glass which can vary its opacity by changing the current passing through it.

Smart glass with varying opacity

Easier and more efficient methods are also being tested to make the production and the recycling of glass more sustainable. Using the heating properties of electro and ferro magnetism, for instance, helps reduce reliance on more primitive heating techniques, decreasing energy consumption significantly, as well as that of carbon admissions.

As we continue to expand our knowledge and catalogue of technology, new innovations will come and old ones will become obsolete.

But one thing is certain: glass will always remain and continue to evolve with us.

Works Cited

E. Alfredo Campo, 3 - Thermal Properties of Polymeric Materials, Editor(s): E. Alfredo Campo, In Plastics Design Library, Selection of Polymeric Materials, William Andrew Publishing, 2008, Pages 103-140, ISBN 9780815515517, (

National Center for Biotechnology Information (2021). PubChem Compound Summary for CID 24261, Silicon dioxide. Retrieved August 2, 2021 from

e-Education Institute. (2021). Why is glass transparent? John A. Dutton e-Education Institute.

Birnie, D. P., & Dyar, M. D. (1986). Cooling rate calculations for silicate glasses. Journal of Geophysical Research: Solid Earth, 91(B4), 509-513.

Miodownik, M. (2014). Stuff matters: Exploring the marvelous materials that shape our man-made world. Houghton Mifflin Harcourt.

Omnexus. (2021). Glass transition temperature (Tg) of plastics - Definition & values. Plastic Materials | Free Online Database for Plastic Industry.

Zenkova, V. (2020, May 5). Is glass biodegradable? A story with a happy ending. EcoFreek - Small Changes, Big Results.

Hardwarezone. (2018, February 5). Halio is a smart glass that can darken on demand within 3 minutes. Technology News, Products & Gadgets Reviews, Latest Tech Updates - HardwareZone Singapore.

Why glass recycling in the US is broken. (2021). Chemical & Engineering News.


Timestravel. (2020, July 27). This is what visiting the Louvre would be like from now on. Times of India Travel.

e-Education Institute. (2021). Why is glass transparent? John A. Dutton e-Education Institute.

Silver, S. (2019, September 30). Why am I watching a Glassblowing show that’s only kind of good? Medium.

Futurity. (2019, March 4). Shattered-glass-circle_1600.

Hardwarezone. (2018, February 5). Halio is a smart glass that can darken on demand within 3 minutes. Technology News, Products & Gadgets Reviews, Latest Tech Updates - HardwareZone Singapore.

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