The illumination produced by a filament bulb arises from the heating of its filament due to the flow of electric current.

When an electric current flows through the filament of a bulb, it encounters resistance. This resistance is inherent to the filament’s material, which is typically tungsten—an imperfect conductor of electricity. As the electrons within the current collide with the atoms of the filament, they transfer some of their kinetic energy to these atoms, resulting in increased atomic vibrations. This process, known as Joule heating (or resistive heating), generates heat.

As the filament’s temperature rises, it begins to emit light due to a phenomenon known as incandescence. Incandescence refers to the emission of light from a hot object as a consequence of its temperature. In typical applications, the filament is heated to extremely high temperatures, often around $3000 \, \text{°C}$, causing it to glow and emit light within the visible spectrum.

The color of the emitted light is dependent on the filament’s temperature, a relationship described by Planck’s law of black-body radiation. This law states that the spectrum of light emitted by a black body, such as the filament, is determined by its temperature. At lower temperatures, the filament primarily emits light in the infrared spectrum, which is invisible to the human eye. As the temperature increases, the emitted light transitions into the visible spectrum, beginning with red at lower temperatures and progressing through to blue at higher temperatures.

Despite producing light, a significant portion of the energy is still lost as heat. This inefficiency is why filament bulbs are not energy-efficient; only about $5\%$ of the electrical energy is converted into visible light, while the remainder is dissipated as heat. Consequently, filament bulbs can become quite hot to the touch after prolonged use. Nevertheless, they remain popular due to their warm light and low cost.