1042. How does a halogen lamp get so hot?
Like all incandescent bulbs, a halogen lamp creates its light as visible thermal radiation from an extremely hot tungsten wire. In fact, the wire in a halogen lamp is allowed to get even hotter than the one in a normal bulb. But while the glass envelope of a normal bulb gets only moderately hot during use, the glass envelope of a halogen bulb gets extremely hot. That's because the halogen bulb is using a chemical trick to keep tungsten atoms from getting away from the filament. Each time one of those tungsten atoms tries to leave, it's picked up by halogen molecules inside the glass envelope and returned to the filament. These halogen molecules can even pick the tungsten atoms up off the glass envelope and return them to the filament, but only if the glass envelope is allowed to get extremely hot. That's why the glass envelope of the halogen bulb is allowed to run so hot—if it weren't, it would accumulate the tungsten atoms permanently and it would darken. And since the tungsten atoms wouldn't be returned the filament, the filament wouldn't last as long.