What defines a black body radiator?

A black body radiator is defined as an idealized physical object that absorbs all incoming radiation, regardless of frequency or angle. This characteristic is fundamental in understanding thermal radiation and is crucial in various fields such as physics and engineering. Since a black body absorbs all incident energy, it does not reflect or transmit any of that energy, making it the most efficient emitter of radiation as well.

In thermal equilibrium, a black body not only absorbs energy but also re-emits energy with a characteristic spectrum dependent solely on its temperature, following Planck's law. This property is used as a reference point for comparing the emissive properties of real materials, which never achieve 100% efficiency.

This definition plays a significant role in the study of thermodynamics and heat transfer, allowing scientists and engineers to model and predict how materials will behave in varied thermal environments.