Basically, an incandescent Bulb is a simple device - just a tungsten wire sealed in a glass tube. The electric current passing through the wire heats it to incandescence, and the wire produces light. Suppose you took a dark piece of metal. If you heat it in a furnace, it becomes hot and radiates heat, but at first shows no visible effects. As temperature is increased, it turns a dull red, therefore emitting not only heat, but also some light. As the temperature is increased further, it turns to a cherry red, then yellow, and at temperatures of about 4600 F, it radiates a bright, nearly 'white-hot' light. For the incandescent lamp, energy is supplied electrically to make up for the energy, which is radiated as heat and light.