one physics question

This is incandescent light emission. The physics principle involved is "black body readiation". Basically, all objects emit electroamgentic waves when heated. When the temperature reaches above 470 degrees Celsius, the electromagnetic waves emitted fall within the visible spectrum, and the object appears red-hot.

In an incandescent light bulb (tungsten filament light bulb), a filament is heated to a temperature above thousands of degrees Celsius. At this temperature, a fraction of the electromagnetic radiation falls in the visible spectrum. The majority of the electromagentic radiation however, is emitted in the infrared part of the spectrum.

Incandescence arises because of rapid vibration of molecules in the heated solid substance. There is a tendency for the energy of these vibrating molecules to lower their energy through emission of electromagentic waves. Since the vibrational energy of these molecules follows a statistical distribution, the electromagnetic energy so emitted would thus compose of all possible frequencies. A continuous spectrum is therefore observed.

You may refer to the following web-page for the expanation of incandescence.

http://en.wikipedia.org/wiki/Incandescence