What defines a black body in physics?

A black body in physics is defined as an idealized physical object that absorbs all incoming electromagnetic radiation, regardless of the frequency or angle of incidence. This means that it does not reflect or transmit any radiation; all light that strikes it is absorbed. The concept of a black body is important in thermodynamics and quantum mechanics because it serves as a perfect emitter and absorber of thermal radiation.

In addition to absorbing radiation, a black body also emits radiation in a characteristic spectrum, known as black body radiation, which depends on its temperature. This emission can range across a wide spectrum from infrared to visible light and even beyond, following Planck's law.

The other options do not adequately describe the concept of a black body. For instance, an object that reflects all electromagnetic radiation would not serve as a black body, as it would not absorb any energy. Similarly, an object emitting only infrared radiation does not encompass the full range of wavelengths a black body would emit based on its temperature. Lastly, while temperature is an important aspect of black bodies, the defining feature is their ability to absorb all radiation rather than simply having a fixed temperature. Therefore, the correct definition is that a black body absorbs all electromagnetic radiation.