What is the structure of incandescent light bulbs?« Back to Questions List

We use light bulbs everyday but probably never had time to look into its internal parts.  The word ‘incandescence’ means emission of light by a body because of its high temperature. The bulb is made of glass filled with the inert gas argon (Ar) inside. There is a very thin metal filament inside the bulb connected on both sides by two wires. These wires are the anode and cathode connected to a metal contact (electrical foot contact in the diagram). 


incandescent ,argon

When the metal contact is supplied with power, charges flow between cathode and anode. This happens via the thin filament.  The filament is deliberately made thin so that it provides more resistance to the current flowing through it (Resistance being inversely proportional to the area of cross section) and thereby producing heat. When the filament gets heated to a very high temperature, it produces visible light. In a 60 Watts bulb, tungsten filament is about 6.5 feet but its thickness is only about one hundredth of an inch. In order to fit in the small space, it is coiled up.

We know that metals can melt when they reach very high temperature and even catch fire when the conditions are ideal for combustion (for example - presence of oxygen). Tungsten metal has a remarkably very high melting point. Although the filament in the bulb gets heated up to such temperatures, the absence of oxygen stops the filament from burning. The presence of argon being inert in nature ensures the tungsten atoms do not react with them stopping loss of tungsten atoms if otherwise. Thus the life of the bulb is not reduced with filament disintegration.

One big disadvantage of these bulbs is that they give off most of their energy in the form of invisible infrared photons (heat) and only 10% as visible light. Thus it is not very efficient and being replaced by fluorescent lamps, compact fluorescent lamps (CFL), LEDs etc. Why shape of CFL lamp differs from incandescent lamp?   Why does a tube light have a glow delay compared to CFL?


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