ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
Light bulb from a car headlamp. This is a tungsten-iodide bulb, designed to be more reliable and work at higher temperatures than normal light bulbs. In an ordinary light bulb tungsten in the heated filament gradually evaporates, blackening the bulb and causing the filament to thin until it snaps. In a tungsten-iodide bulb the atmosphere is iodine. This combines with evaporated tungsten atoms to form tungsten-iodide, a gas. When this gas is warmed by the filament this molecule decomposes into tungsten and iodine again, redepositing tungsten on the filament.
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