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, enough to cross the gap, and ordinary glass absorbs 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.

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.