The Basic Principle of Incandescence
An incandescent light bulb produces light through a process called incandescence, which is the emission of light by a hot body. Inside the glass bulb, a thin wire filament, typically made of tungsten, is heated to an extremely high temperature by an electric current. As the filament gets hot enough, it glows, emitting light across a spectrum of wavelengths, including visible light.
Role of Electrical Resistance and Heat
When electricity flows through the tungsten filament, its high electrical resistance impedes the flow of electrons. This impedance causes the electrical energy to be converted into thermal energy, heating the filament intensely. The filament's material (tungsten) is chosen for its very high melting point, allowing it to reach temperatures around 2,700–3,300 Kelvin (2,427–3,027 °C or 4,300–5,480 °F) without melting.
Preventing Filament Burnout
To prevent the superheated tungsten filament from rapidly oxidizing and burning out, the glass bulb is either evacuated (creating a vacuum) or filled with an inert gas, such as argon or nitrogen. This inert environment slows down the evaporation of the tungsten, extending the bulb's lifespan. However, even with these measures, tungsten atoms still slowly evaporate and deposit on the inside of the glass, causing the bulb to darken and eventually fail.
Energy Efficiency and Spectrum
Incandescent bulbs are relatively inefficient at producing visible light, as a significant portion of the electrical energy is converted into heat (infrared radiation) rather than light. Their emitted light spectrum is continuous, with a larger proportion of red and yellow wavelengths, giving them a 'warm' light appearance. This energy loss is why they are being phased out in favor of more energy-efficient lighting technologies.