Blackbodies are called blackbodies because they absorb all incident radiation and appear perfectly black. When an object absorbs all incoming radiation across the entire electromagnetic spectrum and reflects or transmits none of it, it appears black to our eyes. This is why blackbodies are named as such.
In the context of blackbody radiation, a blackbody is an idealized object that absorbs all radiation incident upon it, regardless of the wavelength or frequency. It absorbs and re-emits radiation in a manner determined solely by its temperature. When a blackbody is in thermal equilibrium, its emitted radiation follows a specific distribution known as the Planckian distribution or the Planckian blackbody curve.
The term "blackbody" was first introduced by Gustav Kirchhoff, a German physicist, in the 19th century. Kirchhoff used the concept of a blackbody to explain the behavior of thermal radiation and to understand the emission and absorption properties of materials. The idealized blackbody concept allowed scientists to establish a theoretical benchmark for the behavior of thermal radiation, which is important in various fields of physics, including thermodynamics, astrophysics, and quantum mechanics.
While true blackbodies are theoretical constructs, there are objects in the universe that closely approximate the behavior of blackbodies. For example, stars, particularly those that are dense and hot, such as our Sun, emit radiation that approximates a blackbody spectrum. These objects are often referred to as "blackbodies" in astrophysics, even though they may not appear visually black to our eyes.
In summary, blackbodies are called blackbodies because they absorb all incident radiation and reflect or transmit none of it, appearing perfectly black. The concept of blackbody radiation and the associated Planckian distribution provide important insights into the behavior of thermal radiation and have applications in various fields of physics.