How It Works

How thick must glass be to become opaque?

Glass absorbs different amounts of light at different wavelengths. For UV light it is already virtually opaque, but even for the visible part of the spectrum it isn’t perfectly transparent.

An ordinary three-millimetre (0.1-inch) sheet of window glass lets about 91 per cent of light pass. With six millimetres (0.2 inches), you’d only get 91 per cent of that 91 per cent – in other words, 83 per cent – and so on.

If you were able to make a sheet of glass a metre (3.3 feet) thick, without introducing any impurities or imperfections, the amount of light making it all the way to the other side would be just 0.002 per cent, which is enough to make full daylight as dim as a moonlit night. However, it still wouldn’t be totally opaque.

Answered by Luis Villazon