Good absorbers are not always good emitters. In fact, there is no law of nature that states that a good absorber must be a good emitter. However, there is a physical principle that explains why some materials that are good absorbers are also good emitters.

The principle is known as Kirchhoff's law of thermal radiation, which states that the emissivity of a material is equal to its absorptivity at the same temperature and wavelength. In other words, a material that absorbs well at a certain wavelength will also emit well at that wavelength.

The reason for this is that the same physical processes that allow a material to absorb electromagnetic radiation also allow it to emit electromagnetic radiation. When a material absorbs a photon, the energy of the photon is converted into thermal energy, which causes the material to heat up. When the material gets hot enough, it will begin to emit thermal radiation in the form of photons.

Therefore, while not all good absorbers are good emitters, there is a physical principle that explains why some materials that are good absorbers are also good emitters.