Anti-radiation missile was achieved by the SAM site's radar receiver locking on to radio noise emissions generated by an aircraft's jamming pod. In cases of heavy jamming, missiles were often launched exclusively in this mode; this passive tracking meant that SAM sites could track targets without needing to emit any radar signals, and so American anti-radiation missiles could not be fired back in retaliation. Recently, the People's Republic of China developed the FT-2000 system to counter AEW and AWACS targets. This system is based on the HQ-9, which is in turn based on the S-300PMU. These anti-radiation missile systems have been marketed
Refraction of incidence "θ" and angle of refraction "θ" is equal to the ratio of phase velocities ("v" / "v") in the two media, or equivalently, to the indices of refraction ("n" / "n") of the two media. Optical prisms and lenses utilize refraction to redirect light, as does the human eye. The refractive index of materials varies with the wavelength of light, and thus the angle of the refraction also varies correspondingly. This is called dispersion and causes prisms and rainbows to divide white light into its constituent spectral colors. Consider a wave going from one material to another where