What Doppler variant reverses the usual frequency shift for approach and recession?

The Doppler variant that reverses the usual frequency shift for approach and recession is the inverse Doppler effect. In an ordinary Doppler shift, an approaching source is observed at a higher frequency and a receding source at a lower frequency.

The inverse version can occur in unusual wave-propagating materials, especially media with negative refraction. In such a medium, the frequency shift can have the opposite sign from the conventional effect: approach may produce a lower observed frequency, while recession may produce a higher one.

The possibility was discussed theoretically by scientists including Victor Veselago from the late 1960s onward. The first experiment reported to detect the effect was conducted in Bristol, United Kingdom, by Nigel Seddon and Trevor Bearpark in 2003. This is not the same as the photoelectric or Compton effects, which concern interactions between light and matter, nor the Zeeman effect, which involves spectral-line splitting in magnetic fields. The inverse Doppler effect depends strongly on the properties of the medium.