Definition of a Black Body
A black body is an idealized physical object that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. It neither reflects nor transmits any radiation, meaning it would appear perfectly black if it were at room temperature and did not emit its own light.
Ideal Emitter of Radiation
Despite its name, a black body is also an ideal emitter. It radiates electromagnetic energy at every frequency according to its temperature. The spectrum of this emitted radiation, known as black-body radiation, is characteristic only of the body's temperature, not its composition or shape.
Real-World Approximation
While a perfect black body does not exist in reality, a small opening into a hollow cavity or oven can closely approximate one. Any radiation entering the small hole is almost entirely absorbed, as it undergoes multiple reflections and absorptions within the cavity before having any chance to escape.
Significance in Physics and Quantum Mechanics
The study of black bodies was pivotal in the development of quantum mechanics. Classical physics failed to accurately explain the observed black-body radiation spectrum, leading Max Planck to propose the revolutionary concept of energy quantization in 1900, fundamentally changing our understanding of energy at the atomic level.