Kirchhoff's law of thermal radiation says that:
For a body of any arbitrary material, emitting and absorbing thermal
electromagnetic radiation at every wavelength in thermodynamic
equilibrium, the ratio of its emissive power to its dimensionless
coefficient of absorption is equal to a universal function only of
radiative wavelength and temperature, the perfect black-body emissive
power.
I can imagine why dark objects have higher absorption of electromagnetic radiation: The darker the object is the less radiation it reflects back. An ideal black body would absorb all the incident electromagnetic radiation.
Is there a similarly simple and intuitive explanation of why dark objects emit thermal electromagnetic radiation faster that light objects and why is the Kirchhoff's law valid? For me it is not intuitive at all and I was not able to find any simple explanation.
Best Answer
Generally speaking solids absorb light by converting the EM radiation to lattice vibrations (i.e. heat). The incident light causes electrons in the solid to oscillate, but if there is no way for electrons to dissipate the energy then electrons will simply reradiate the light and the light is reflected.
In metals the transfer of energy from oscillations of the conduction electrons to lattice vibrations is slow, so the light is mostly reflected. By contrast in graphite the light is absorbed by exciting $\pi$ electrons, and the excited orbitals efficiently transfer energy to the bulk so the light is mostly absorbed.
But as dmckee says in his comment, the microscopic physics is reversible. If it's hard for oscillating electrons to transfer energy to bulk lattice vibrations then it's equally hard for those lattice vibrations to transfer energy back to the electrons and hence back out as light. So a shiny metal will be equally bad at absorbing and emitting light.
Similarly, in graphite if coupling of $\pi$ orbitals to lattice vibrations is efficient then energy flows equally fast both ways, and graphite will be equally good at absorbing and emitting light.
In practice black body radiation is a mish mash of all sorts of different mechanisms, and the two cases I've mentioned are just examples. However in all cases when you look in detail at how energy is being transferred you'll find it's a reversible process and the energy flows equally fast in both directions.