How Does Glass Let Light Through? The Science Explained

It's a lovely summer weekend morning. We relax in our living room with the sunlight coming gently through the windows. We glance outside and can see the green leaves and watch birds in the yard. We can do this thanks to the windows we have in nearly every room in our home. We see them in our neighbors' homes, in stores and restaurants, in office buildings, and more. They are so commonplace in our lives that we don't stop to think about how they work the way they do at all.

Home windows as we know them today came about in 17th-century Britain. Glass manufacturing developed extensively to get to that point, and it has come a long way further to bring us to today where windows are strong, regularly used, and perfectly see-through. And glass research continues to develop, like the new glass-like material that can clean itself.

The reason why glass can let light through has to do with the way it is made from sand, turning it into an amorphous solid. The amorphous quality is the fundamental reason why light shines through glass but not wood (though transparent wood may be just over the horizon).

Glass is an amorphous solid, which you get when a solid substance is melted and then quickly cooled. Because of this manufacturing process, the atoms and molecules are arranged randomly, much in the same way they are in water, which you can also see through.

In materials like glass, electrons require a lot of energy to jump between energy levels, and visible light photons just aren't strong enough to trigger that jump. Instead of being absorbed or reflected, the light simply passes through. That's why we can see through glass, because it's transparent to visible light.

A professor at the University of Nottingham, Philp Moriarty, explained it to the Guardian, "The reason you can't look through a sheet of paper but easily through centimetre-thick glass [is that] the glass is amorphous, so it is not ordered, but it doesn't have very many defects. At the scale of the wavelength of light, the glass is uniform. The paper consists of lots of fibres and those fibres – the sizes, the diameters, the widths, the spacings between them – are not that different from the wavelength from light and [so] they scatter the light. The upshot is a snowy, opaque sheet on your sketchpad."

To understand even deeper why glass is transparent, it's important to fully explain how it is made. Glass begins life as sand. Not just any sand is chosen, but sand with quartz crystals in it. The sand gets heated to over 3,000 degrees Fahrenheit in order to melt it, causing the quartz to lose its crystalline structure. As it cools, It then into an amorphous solid, so from a molecular standpoint, it is between a solid and a liquid.

After it's melted, it must be shaped and made strong enough to become your windows. This is achieved by cooling it down very quickly. This entire process can be long and laborious, so there are shortcuts to speed up production. Adding sodium carbonate to sand lowers its melting point, reducing the energy needed to make glass, but also decreases its chemical durability. To counter this, calcium carbonate is added as a stabilizer to improve the glass' resistance.

It's this history of innovation, and the efforts to improve glass, that allow us to enjoy sunlight through our windows today without being subjected to the elements and other critters that might scamper in. As research continues to be done, like self-healing glass that can repair itself with water, it is exciting to think about how glass will evolve in the coming decades.