Why is incandescent light not monochromatic or coherent?

Incandescent light is not monochromatic or coherent primarily because the heating of the filament generates a wide range of frequencies. When the filament is heated to high temperatures, it emits light across a spectrum rather than at a single wavelength. This is due to the thermal radiation emitted by the hot filament, which produces light that includes many different wavelengths corresponding to different colors.

Monochromatic light consists of a single wavelength and corresponds to a single color, while coherent light refers to waves that maintain a constant phase relationship. Since incandescent light results from a range of thermal emissions, it contains multiple wavelengths simultaneously, leading to the conclusion that it is neither monochromatic nor coherent. The broad spectrum produced means that the light waves do not have the same frequency or phase relationship required for coherence.