text: The propagation of radiation in a medium that is assumed to be homogeneous, isotropic, and at rest takes place in straight lines and has the same velocity in all directions. Unless if propagating through a vacuum, thermal radiation does decay over time as energy is scattered. 
  Scattering occurs due to the presence of discontinuities in every medium that arise from their atomic structure. An example of scattering is when thermal radiation from the sun scatters after entering the earth's atmosphere. On a clear day at noon, only about two-thirds of this radiation actually reaches the surface. The rest is intercepted by particles in the air and changed into heat in the process. Scattering is noticeably larger for rays of shorter wave length; hence the blue color of skylight.