What does the term 'black body' refer to in optics?

The term 'black body' in optics refers to an idealized physical object that is a perfect emitter and absorber of electromagnetic radiation, including light, at all wavelengths. Such a body absorbs all incident radiation, regardless of frequency or angle, and does not reflect any light. This characteristic makes it the ideal standard for measuring the emissive properties of real materials.

In thermal equilibrium, a black body will emit radiation known as black body radiation, which depends solely on its temperature and is described by Planck's law. This emission is purely thermal and provides a foundation for understanding concepts like temperature and thermal radiation in fields such as thermodynamics and quantum mechanics.

While some options suggest varying characteristics, a black body is specifically defined by its perfect absorption and emission without any reflections or selective absorption, illustrating its unique role in optical physics and related applications.