by Bernie Lyon
Referring to Figure 1, the “ideal” block has an emissivity of 0.2 and a reflectivity of 0.8 We are assuming that these values are constant and do not change with angle. This is not the case. If thermal radiation strikes a single surface, 20% of the radiation is absorbed and 80% is reflected. (Keep in mind 20% of the radiation – Not the temperature!) If it strikes another surface before exiting, 20% of what remains is absorbed and 80% of that is reflected, and so on.
Therefore, with multiple reflections caused by cavities, the effective emissivity, or emittance, increases, while the effective reflectivity decreases. This is the reason why holes and cavities appear dark. They reflect less visible light.