My physics teacher told that infrared waves are not passing through walls due to rayleigh scattering,but rayleigh scattering increases with increase in wavelength,then why radio waves are passing through walls so easily

  • $\begingroup$ I would doubt it is actually Rayleigh scattering, that makes infrared radiation not pass through walls. But the Rayleigh scattering cross section decreases with wavelength, as is is $\propto f^4$ (that's why the sky is blue and the setting sun is red: the blue light is Rayleigh scattered, while the red light is scattered with a much lower cross section). $\endgroup$ – Sebastian Riese Jun 4 '18 at 13:57
  • $\begingroup$ "rayleigh scattering increases with increase in wavelength" No, that's wrong. $\endgroup$ – Bill N Jun 4 '18 at 14:51
  • $\begingroup$ then what would be the possible reason not allowing infrared to pass through walls,not resonance with vibrational frequencies of molecules in wall for the whole band of infrared $\endgroup$ – user197535 Jun 4 '18 at 15:07
  • $\begingroup$ @sebastian riese what could be the possible reason for infrared not passing through walls,please reply. $\endgroup$ – user197535 Jun 4 '18 at 16:29
  • $\begingroup$ did you understand the answer to your other question about infrared? $\endgroup$ – anna v Jun 14 '18 at 14:50

You can calculate the transmission coefficient from the dielectric constant and thickness of a slab of homogeneous material. The dielectric constant of the basic materials (wood, plaster, brick) should have little or no imaginary part at RF, but embedded metal components (nails, wires, mesh, studs) will reflect and/or scatter the waves. At IR wavelengths, the dielectric constant has a large imaginary part, due (just as you said) to the vibrational modes of the molecules, which give rise to broad absorption bands because of collisions and/or interactions with phonons.

Raleigh scattering is a whole nother phenom in a whole nother regime. As designers of weather-detecting radars well know, the radar cross section of a raindrop or other quasi-spherical target scales as $\sigma \propto {{R}^{6}}/{{\lambda }^{4}}$ when $\lambda \gg R$ but levels off (aside from some subtle ripples) when $\lambda >R$.


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