How Everything Works
How Everything Works How Everything Works

Question 662

Why does a gas lantern use a silk mantle? How does it produce such intense light — BW, Santa Clara, CA
The mantle of a lantern is actually a ceramic ash. The silk itself burns away completely and leaves behind only of the oxides of materials that were incorporated in the silk mantle when it was manufactured. The principal oxide formed when the standard Welsbach mantle is burned is thorium oxide, with a few percent of cerium oxide and other oxides. This use of thorium oxide or thoria, is a rare example of a radioactive element (thorium is radioactive) permitted in common household use. Thoria glows brightly when heated because it can tolerate extremely high temperatures without melting and because it is a very effective emitter of thermal radiation at temperatures of roughly 2200° C.

The light emitted by these oxide mantles is shorter in average wavelength than can be explained simply by the temperature of the burning gases, so it isn't just thermal radiation at the ambient temperature. The mantle's unexpected light emission is called candoluminescence and is thought to involve non-thermal light emitted as the result of chemical reactions and radiative transitions involving the burning gases and the mantle oxides.


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