Re: Why can't infrared waves pass through walls while radio waves can?

Higher frequency is often associated with higher penetration power since 
the energy of electromagnetic waves is proportional to frequency. Radio 
waves are relatively low frequency, long wavelength emissions typically 
ranging from a few millimeters to a meter. The visible spectrum has 
higher frequency and shorter wavelengths, ranging from about 400 
nanometers (0.0004 millimeters)to a little over 900 nanometers. The 
infrared spectrum span the region between radio and visible, with 
wavelengths that range from about 1000 nanometers to about 1 millimeter.

Although infrared wavelengths are shorter than radio, they are still 
quite large as compared to atomic spacing and their energy is still small 
compared with the electrostatic force of the atomic electrons. Therefore, 
both radio and infrared (and visible) interact with matter through its 
molecular structure and macroscopic properties. Radio waves pass through 
walls, this is true, but are slightly attenuated. The chemical makeup of 
the walls permit this - that is they are transparent (or mostly so) to 
radio waves. However, soil in general reflects radio waves. Conductive 
materials also reflect radio waves via the induced currents - hence radar 
works. The infrared behave similarly. Wood contains large carbon and 
water content and both are excellent absorbers of infrared energy at many 
wavelengths. Some materials, like germanium are (mostly) transparent to 
infrared energy. So the pernetration (or lack thereof) of infrared 
radiation versus radio radiation is a consequence of the chemical and 
physical makeup of the medium - not the penetration of high energy 
radiation. It is not until one reaches the x-ray spectrum or gamma-ray 
spectrum that the wavelengths become small enough and energies become 
great enough to interact with the lattice of atomic nuclei which compose 
the wood, metal,... etc.