Question

How a electric bulb produces photons when electric potential given?

Answer 1

The photons aren’t created! A photon is a quantum of electromagnetic energy. Energy can be converted from one form to another (including into matter and back), but the energy output of the bulb equals the energy input of the bulb. When you turn on the light switch, energy in the form of moving electrons (electricity) is converted into electromagnetic energy. Visible light is a form of electromagnetic energy.

In an incandescent light bulb, the electricity passes through a very thin wire. All the moving electrons get crowded together on the outside of this thin wire. When electrons become crowded they bump into each other. They also bump into the electrons in the metal of the wire and some of their energy is transferred to the metal atoms. Heat is nothing more than atoms that are bumping into each other. So the thin wire gets extremely hot—so hot it glows white. The heat is converted not only into light but also infrared, microwave radiation, radio waves, and a teeny bit of ultraviolet light on the high end. These are all merely photons containing different amounts of energy. All of the ultraviolet, infrared, microwave, and radio wave energy is considered wasted because it isn’t producing what you wanted in the first place: Visible light.

In a florescent bulb, the electricity is passing through mercury vapor held inside a glass tube. The flowing electrons from the electric circuit hit some of the electrons in the vapor. Those electrons in the mercury atoms become “excited”. They don’t stay excited very long. Pretty soon they emit a photon of ultraviolet light. Ultraviolet photons have more energy than visible light photons. This ultraviolet light photon then hits the phosphorescent coating on the inside of the bulb. The ultraviolet light excites the electrons in the chemicals that make up the coating. These chemicals emit photons of lots of different colors. A mixture of lots of different colors is what white light is. The difference in the energy of the ultraviolet photons and the visible light photons is wasted as heat.

In an LED “bulb” the electricity passes through a semiconductor known as a diode. The manufacturer of the semiconductor put some impurities in it that act like little holes. You can think of these holes as places for the electrons to “fall down” as they pass through the semiconductor. A good analogy is a cliff with a waterfall. The cliff is the hole and the flowing water is the moving electrons. The “depth” of the holes (the height of the cliff) is carefully chosen so that when an electron “falls down” one of the holes, the amount of energy it releases corresponds to a photon of visible light. Very little of the electricity is wasted as heat which is why LED bulbs are so energy efficient.