|MadSci Network: Physics|
The difference between radio wave and light is that the oscillation of radio wave is much slower than light. Generally radio waves are below GHz (10^9 Hz) while lights are above THz (10^12 Hz). When electromagnetic wave incident on flat materials, part of it will be reflected back, and part of it will penetrate into the material. When wave propagates through a material, it will attenuate. The reflection depends on the permittivity of the material. The greater the permittivity, the greater the reflection. The attenuation of the wave during propagation in the material depends on the loss of the material. Both the permittivity and loss of a material depends on frequency of the wave. As to the material of the walls, generally their loss is much higher at light frequency than at radio frequency, therefore even if most of the light penetrates into it (in cases when the wall looks dark), the light will soon be attenuated to zero (the energy is absorbed by the wall and turned into heat). And in some cases much of the light is reflected back (in cases when the wall looks bright). As to what determines the permittivity and loss of material. They are related to the molecular or atomic or crystal structure of the material. You can imagine the electrons in material as small charged spheres connected to the kernel with springs or rubber bands. When electromagnetic waves come, they will push these spheres back and force, and the movement of the spheres caused emission of secondary electromagnetic waves, which determine the permittivity of the material. These spheres have intrinsic resonant frequencies at which very small push will cause very large movement. These frequencies correspond to the frequencies where the permittivity and loss is large. Far from these resonant frequencies, the permittivity and loss are small. The wall material has some resonant frequencies in the light frequency range, that's the reason why it absorbs light.
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