Why Is Glass Transparent?

Have you ever observed the construction of a house? Builders start by framing the structure with wooden studs, followed by attaching plywood sheathing to create walls. These walls often feature window openings, each fitted with a glass pane encased in a frame. Windows infuse homes with brightness, warmth, and a welcoming ambiance by enabling light to enter. But what makes glass transparent compared to the surrounding wood? Both materials are solid and serve as barriers against rain, snow, and wind. However, wood is opaque, blocking light entirely, while glass remains transparent, allowing sunlight to pass through unobstructed.

Some individuals, and even certain science textbooks, attempt to explain this phenomenon by claiming wood is a true solid, while glass is a slow-moving liquid. They argue that the atoms in glass are spaced farther apart, creating gaps that permit light to pass. They might also cite the wavy, uneven appearance of old house windows as proof that glass has "flowed" over time, akin to molasses moving slowly on a chilly day.

In truth, glass is not a liquid. It belongs to a unique category of solids called amorphous solids. This state of matter features atoms and molecules fixed in place but arranged randomly, unlike the orderly structure of crystals. Consequently, glass exhibits the rigidity of a solid while maintaining the molecular disorder of a liquid. Amorphous solids are formed by melting a solid at high temperatures and then cooling it rapidly, a process referred to as quenching.

Glasses share numerous similarities with ceramics, possessing qualities such as durability, strength, brittleness, high resistance to electricity and heat, and minimal chemical reactivity. Oxide glass, commonly used in sheet glass, containers, and light bulbs, boasts an additional key feature: it allows visible light wavelengths to pass through. To grasp why this occurs, we need to examine the atomic makeup of glass and explore how photons — the fundamental particles of light — interact with this structure.

We’ll explore this in detail next.

Electron to Photon: You Don't Excite Me

To begin, remember that electrons orbit the nucleus of an atom, occupying distinct energy levels. For an electron to transition to a higher energy level, it must absorb energy. Conversely, to drop to a lower energy level, it must release energy. In both scenarios, energy exchange occurs in specific, quantized amounts.

Now, imagine a photon approaching and interacting with a solid material. Three possible outcomes can occur:

Glass belongs to the third category. Photons travel through it because they lack the energy needed to elevate a glass electron to a higher energy level. Physicists often explain this using band theory, which describes energy levels grouped into energy bands. Between these bands are band gaps, areas where no electron energy levels exist. Glass has a particularly large band gap, meaning its electrons require significantly more energy to transition between bands. Photons of visible light — with wavelengths between 400 and 700 nanometers, covering colors like violet, blue, green, yellow, orange, and red — don’t possess enough energy to trigger these transitions. As a result, visible light photons pass through glass without being absorbed or reflected, rendering it transparent.

At wavelengths shorter than visible light, photons gain sufficient energy to shift glass electrons between energy bands. For instance, ultraviolet light, with wavelengths from 10 to 400 nanometers, cannot penetrate most oxide glasses, such as window panes. This makes windows, including those in our imagined house under construction, as impenetrable to ultraviolet light as wood is to visible light.

Continue reading for more resources to brighten your understanding of the world.