Re: Why can radio waves pass through walls and light waves not?

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.