View of an incandescent light bulb. The bulb glows because of a current passing through a thin, coiled tungsten wire. The current heats the wire, making it glow white hot. The bulb is filled with an inert gas at low pressure to prevent oxidation. Incandescent bulbs are very inefficient at converting electricity into light, about 5% is normal, and so have been replaced by more recent technologies such as compact fluorescent bulbs and LED lights.