Why Is Glass Transparent When Sand Is Not? The Physics of Clear Glass

Glass begins as sand, soda and limestone heated in a furnace. Sand is grainy and opaque; a window is so clear that birds sometimes fly into it. How can the same material be both? The answer comes from two different ideas: what light needs to be absorbed, and what makes light scatter.

What glass is made of

Most everyday glass, the kind in windows and bottles, is soda-lime glass. It is about 70 to 74% silica (silicon dioxide, the main component of sand), with soda (sodium oxide) to lower the melting temperature and lime (calcium oxide) to make it more durable.

When this mixture is melted and cooled quickly, the atoms do not have time to arrange themselves into an orderly crystal. They freeze in place in a random network. A material like this is called an amorphous solid.

Reason one: visible light is too weak to be absorbed

Light travels as packets of energy called photons. A material absorbs light when a photon has just the right amount of energy to lift one of its electrons to a higher energy level.

In metals, electrons can absorb almost any amount of energy, so metals absorb and reflect visible light and are opaque and shiny. In silica glass, the electrons are held tightly in chemical bonds. To excite them, a photon would need about 9 electronvolts of energy. Photons of visible light carry only 1.6 to 3.3 electronvolts.

Visible photons therefore cannot be absorbed by the electrons; they simply pass through. Light does slow down slightly inside glass, which is why glass bends light, the effect that makes lenses work.

Reason two: no boundaries to scatter light

If silica is transparent, why is sand not? Sand is made of countless tiny grains, many of them crystalline quartz. Every time light crosses from air into a grain and out again, it bends and partly reflects. After thousands of such encounters in random directions, the light is scattered everywhere. This is why sand, sugar, salt, snow and crushed glass all look white or pale, even though each grain is clear.

A solid piece of glass has no grains and no internal boundaries. Light enters at one surface and leaves at the other, with very little scattering in between.

Frosted glass uses this effect on purpose: roughening the surface creates many tiny facets that scatter light, so the glass lets light through but hides what is behind it.

Glass is not clear to every kind of light

Glass is transparent to visible light, but not to everything.

• Ultraviolet: ordinary window glass absorbs most UVB, the main cause of sunburn, but lets much of the longer-wave UVA through. You are unlikely to get sunburnt through a car window, but UVA can still contribute to skin ageing.
• Infrared: glass lets in short-wave infrared from the sun but absorbs much of the long-wave infrared, the heat radiation given off by warm objects inside a room. This is part of how a greenhouse traps warmth.

The slight green tint you see when looking through the edge of a pane comes from traces of iron oxide, which absorb a little red light. Very pure glass, such as the glass used in optical fibres, is so transparent that light can travel through kilometres of it.

The myth of flowing windows

A popular story says that glass is really a very slow liquid, and that is why old cathedral windows are thicker at the bottom. It is not true.

In 1998 the Brazilian materials scientist Edgar Zanotto calculated how long medieval glass would take to flow noticeably at room temperature. The answer was far longer than the age of the universe. The uneven thickness of old windows comes from the way the glass was made: methods such as spinning molten glass into a disc produced panes that were thicker in places, and glaziers often installed the heavy edge at the bottom.

In short

• Glass is mostly silica whose atoms are frozen in a random network.
• It is transparent because visible light has too little energy (1.6–3.3 eV) to excite its electrons (about 9 eV needed).
• A solid piece of glass has no grain boundaries to scatter light, unlike sand or crushed glass.
• Window glass blocks most UVB but lets through much UVA.
• Old windows are thicker at the bottom because of how they were made, not because glass flows.