When current is passed through the filament of an incandescent lamp, its temperature rises and it starts to emit electromagnetic radiation.
The spectrum of the wavelength of radiation emitted by a hot body can be estimated using that emitted by an ideal black body. It has been found that when a body is heated, the radiation emitted has a wide range of frequencies with only the highest frequency of radiation emitted dependent on the actual temperature of the emitter.
When the filament of the bulb is lit, most of the radiation lies in the infra-red spectrum. Only a small portion of the radiation has a frequency range lying in the visible spectrum. The rest is infra-red radiation or heat.