Which object is theoretically able to completely absorb all incident radiation and emit all that absorbed energy?

The correct answer is a black body. A black body is an idealized physical object that perfectly absorbs all incoming radiation, regardless of wavelength or frequency. It is also defined as an ideal emitter of radiation; when it absorbs energy, it can emit that energy as thermal radiation with 100% efficiency. This means that no energy is reflected or transmitted, leading to a perfect absorption rate.

In the context of thermal radiation, a black body serves as a standard because it has a unique relationship between its temperature and emitted radiation, described by Planck's law. As the temperature of a black body increases, it emits more radiation and at shorter wavelengths, which is a fundamental concept in thermodynamics and radiation physics.

Other options represent different concepts: a gray body, for instance, absorbs and emits radiation but not at the same efficiency as a black body; it shows a constant absorption coefficient less than one for all wavelengths. A selective radiator is one that emits energy at specific wavelengths rather than uniformly across all wavelengths, distinguishing it from the theoretical concept of a black body. Lastly, a white body, while it reflects most incident radiation, does not absorb it effectively, making it unsuitable for this definition. Thus, the unique properties of a black body make it the only