Kirchhoff’s law of thermal radiation states that for a body in thermal equilibrium, the emissivity (how well it emits radiation) is equal to its absorptivity (how well it absorbs radiation) at each wavelength.
Key insight:
- A good absorber of radiation at a given wavelength is also a good emitter at that wavelength.
- A perfect blackbody, which absorbs all incoming radiation (absorptivity = 1), also emits the maximum possible radiation at every wavelength.
Why it matters:
This law provides the foundation for understanding blackbody behavior, spectroscopy, and the interaction between matter and radiation. It explains why objects that appear dark (like soot) are also efficient radiators, and it underpins much of thermal imaging and astrophysics.