Why Is Glass Transparent? A Solid That Never Quite Froze

Most solids are crystals. When a liquid metal or a solution of salt cools, the atoms settle into a regular repeating lattice, and a piece of the material is made of many small crystals with boundaries between them. Glass does not do this. Molten silica, silicon dioxide, is so viscous that as it cools the atoms cannot move fast enough to find their places in a lattice, and they are caught in the disordered arrangement of the liquid, frozen in place. The result is called an amorphous solid: rigid, but with the random structure of a liquid.

That randomness is the first reason for transparency. In a crystalline material the boundaries between grains scatter light in all directions, which is why a block of ice with many crystals is cloudy while a single clear crystal is not, and why a sheet of paper, made of tangled fibres, is opaque although each fibre is translucent. Glass has no grain boundaries because it has no grains; it is one continuous disordered network, uniform at the scale of a light wave, so a light wave passes through it without being scattered.

Uniformity is not enough on its own; a material also has to fail to absorb the light. Light is absorbed when a photon's energy matches the energy needed to lift an electron from one allowed level to a higher one. In metals the electrons can take almost any energy, which is why metals are opaque and shiny. In silica the electrons are tightly bound, and the gap to the next available level is large, about nine electron volts, while a photon of visible light carries between about 1.8 and 3.1. A visible photon simply does not have enough energy to move an electron, so nothing absorbs it, and it continues on its way.

Ultraviolet photons carry more energy, and the soda, lime and iron traces in ordinary glass absorb them, which is why you cannot get sunburnt through a window. Infrared photons at the other end are absorbed by the vibration of the atoms, which is why glass traps heat in a greenhouse. Transparency is a property in a particular range of wavelengths, and glass happens to be transparent in the range human eyes use, which is not a coincidence, since eyes evolved to use the wavelengths that pass through water and air.

A grain of quartz sand is chemically the same silica, and a single crystal of quartz is as transparent as glass. Sand is opaque because it is millions of tiny grains, each with surfaces that reflect and refract light, so light entering a handful of sand is bounced in every direction and comes back out as white. Melting the sand fuses the grains into one continuous piece with no internal surfaces, and the light goes straight through. Frosted glass is made opaque again by roughening its surface, and a shattered windscreen turns white for the same reason.

Pure silica melts at about 1,700 degrees, too hot for ordinary furnaces, so glassmakers have added other things since the Bronze Age:

• •Soda, sodium carbonate, which lowers the melting point to around 1,000 degrees, and lime, which stops the result dissolving in water; soda-lime glass is windows, bottles and jars
• •Boron oxide, which gives borosilicate glass that barely expands when heated and so survives the oven and the laboratory
• •Lead oxide, which raises the refractive index and gives crystal glass its sparkle
• •Metal oxides for colour: iron for green, cobalt for blue, gold for ruby red, manganese to cancel the green of iron impurities
• •Almost nothing, for the fused silica of telescope mirrors and the fibre-optic cables that carry the internet, which must be so pure that light survives a hundred kilometres of it

Glass is the material of seeing. Clear window glass, made in quantity from the Middle Ages, let light into buildings and weather out. Spectacles, from the thirteenth century, doubled the working life of scholars and craftsmen. The lens gave the telescope and the microscope in the same decade around 1600, and with them the moons of Jupiter and the cell. The mirror, the barometer, the thermometer, the vacuum tube, the light bulb, the camera and the screen you are reading on all depend on a solid that lets light through, and the internet runs on glass threads thinner than a hair. It is the one material that lets an observer see what is happening without being part of it.

A popular story says glass is a very slow liquid, and that proof can be seen in old church windows, whose panes are often thicker at the bottom. It sounds convincing but is wrong. Early window glass was made by spinning molten glass into a large disc or by blowing and flattening cylinders, and both methods produced sheets of uneven thickness. Glaziers often put the heavier edge at the bottom, but not always, and some old panes are thicker at the top or the side, which a slow flow could never produce.

Calculations based on how the viscosity of glass changes with temperature suggest that at room temperature it would take vastly longer than the age of the universe for a window to sag noticeably. Roman glass vessels and ancient lenses have kept their shapes for two thousand years, and old telescope lenses still focus sharply. Glass is called a liquid only in the sense that its atoms keep the disordered arrangement of one. In every practical sense, including how it behaves over centuries, glass is a rigid solid, and the unevenness of old windows records how they were made.

Look at a sheet of ordinary window glass face on and it seems colourless, but look at its cut edge and it is distinctly green. Nothing about the glass changes; only the distance the light travels through it does. Ordinary glass contains a trace of iron, which absorbs a little light towards the red end of the spectrum and some at the blue end. Across a few millimetres the loss is too small to notice, but along the edge the light passes through many centimetres and the losses add up. That is why makers of aquariums, display cases and solar panels pay extra for low-iron glass.

The same arithmetic set the limits of fibre optics. In 1966 the engineer Charles Kao argued that glass fibres lost light mainly because of impurities, not because of glass itself, and that purified glass could carry signals over useful distances. Corning made such a fibre in 1970, and modern fibre, carrying infrared light at wavelengths where silica absorbs least, loses only around 4 or 5 percent of its light per kilometre. After about 15 kilometres half the light remains, after 100 kilometres about a hundredth, and amplifiers along the cable restore the signal. Kao shared the Nobel Prize in Physics for the idea in 2009.

Ordinary glass is strong when squeezed but breaks easily when pulled, because tiny flaws on its surface open into cracks under tension. Toughened glass works by building compression into the surface. Thermally tempered glass is heated and then cooled quickly with jets of air, so the outside sets first and ends up squeezed by the interior, and when it does break it shatters into small, blunt pieces.

Phone screens usually use chemically strengthened glass instead. The glass is soaked in a bath of molten potassium salt, and larger potassium ions swap places with smaller sodium ions near the surface. The bigger ions are crammed into spaces meant for smaller ones, creating a compressed surface layer that resists scratches and cracks, which is why modern screens survive drops that would shatter a window pane.

Glass is transparent because it is an amorphous solid, a liquid structure frozen without crystal grains to scatter light, and because the electrons in silica are bound so tightly that visible photons lack the energy to be absorbed, though ultraviolet and infrared are. Sand, the same silica in millions of grains, is opaque only because of its surfaces, and melting it into one piece is what lets the light through.