What causes the glow in a filament bulb?

The glow in a filament bulb is caused by the heating of the filament due to the passage of electric current.

When an electric current is passed through the filament of a bulb, it encounters resistance. This resistance is due to the filament's material, typically tungsten, which is not a perfect conductor of electricity. As the electrons in the current collide with the atoms of the filament, they transfer some of their kinetic energy to the atoms, causing them to vibrate more vigorously. This increased vibration of the atoms manifests as heat, a process known as Joule heating or resistive heating.

As the temperature of the filament rises, it begins to emit light. This is due to a phenomenon known as incandescence. Incandescence is the emission of light from a hot body as a result of its temperature. The filament of a bulb is heated to such high temperatures (typically around 3000 degrees Celsius) that it begins to glow, emitting light in the visible spectrum.

The colour of the light emitted depends on the temperature of the filament. This is described by Planck's law of black-body radiation, which states that the spectrum of light emitted by a black body (in this case, the filament) depends on its temperature. At lower temperatures, the filament emits light in the infrared spectrum, which is not visible to the human eye. As the temperature increases, the light emitted shifts into the visible spectrum, starting with red at lower temperatures and moving through to blue at higher temperatures.

However, most of the energy is still wasted as heat. This is why filament bulbs are not very energy efficient: only about 5% of the electrical energy is converted into visible light, with the rest being dissipated as heat. This is also why filament bulbs can become very hot to touch when they have been on for a while. Despite their inefficiency, filament bulbs are still widely used because of their warm light and low cost.