Thermal Radiation, Black Bodies, and the Greenhouse Effect

The final theme of this unit connects energy production to the physics of thermal radiation and the climate. Every object at a temperature above absolute zero emits electromagnetic radiation because of the thermal motion of its charged particles. To analyze this precisely, physicists use the idealized concept of a black body: a perfect emitter and perfect absorber that absorbs all radiation falling on it and reflects none. A black body emits a characteristic spectrum of radiation that depends only on its temperature. Stars, including the Sun, and the Earth itself behave approximately as black bodies, which is why this idealization is so useful in astrophysics and climate science.

Two laws describe black-body radiation. The Stefan–Boltzmann law gives the total power (luminosity) radiated by