Incandescent light bulbs are the oldest types of electric lights and are still in common use today. Incandescent lights get hot very quickly, and they can easily burn your hand if you touch them. Incandescent lamps are actually hot enough to catch paper and some types of cloth on fire if they are directly in contact with the bulb.
Incandescent light bulbs consist of a glass tube with a narrow tungsten wire inside called a filament. The tungsten filament is connected to two electrical contacts, which cause electricity to flow through it. The bulb has either a vacuum inside or an inert gas such as argon or xenon.
The filament is a resistor: a device that resists the flow of electricity. When electricity flows through a resistor, it turns from electric energy to heat energy.
Heat and Radiation
When something heats up, its electrons become excited and jump to higher, less stable energy levels. These electrons will release some of this energy as photons of light.
The heat of an object determines the amount of light it produces and the wavelength of that light. To emit bright white light, the filament in a bulb has to heat up to 2,000 degrees. Much of that heat is radiated out of the bulb, which is why the surface is so hot.
LEDs and CFLs are newer types of light bulbs that are starting to replace incandescent bulbs. They produce far less heat and are not hot to the touch.