Transparency of light and electromagnetic waves

[ThomasGR]

Hello,

For reasons I’ve read here in this forum, some material (let’s say PVC or paper) are not transparent to light. But light is an electromagnetic wave. If I lower the frequency, that material is now transparent, even if I cannot detect it with my eyes but must use special equipments, the same if I increase the frequency. It seems only in the bandwidth of visible light, that material in not transparent. Can some one tell me the reasons?

Best regards.

 

[sophiecentaur]

Hello and welcome.
This is a complicated topic because of the vast range of EM waves and there are several different effects observed. Reflection, absorption and refraction (bending) and scattering will all occur to some extent, always.
All materials have different transmission characteristics for different wavelengths of EM. The performance for the optical wavelength range will be different from the performance for other wavelengths but there is nothing essential special about light . The reason why a material absorbs or reflects a particular wavelength is due to what happens to the electrons in the substance. In metals, there are some very mobile electrons (good conductivity) and the currents induced in the surface cause high reflection (shiny and reflect radio waves) of all wavelengths down to X rays, where the photons interact differently with the lattice.
For insulators (glass, plastic etc) the electrons are not so mobile and will interact to a greater of lesser extent with different wavelengths. If they do not interact (much), the material looks transparent but a small percentage of energy is always absorbed on the way through.
The more dense the material (lead glass compared with ordinary glass) the more the interaction and the more the em path can be bent. Electrons in the material will be induced to vibrate a bit and this loads the wave, slowing it down (it no longer travels at c). For some substances, the movement of the electrons also causes energy to be absorbed at the same time and the material is opaque to that particular wavelength. Coloured filters are opaque to some wavelengths and transmit others.