[2] It was first predicted by Emil Wolf in 1987[3][4] and subsequently confirmed in the laboratory in acoustic sources by Mark F. Bocko, David H. Douglass, and Robert S. Knox,[5] and a year later in optic sources by Dean Faklis and George Morris in 1988.
[6] In optics, two non-Lambertian sources that emit beamed energy can interact in a way that causes a shift in the spectral lines.
It is analogous to a pair of tuning forks with similar frequencies (pitches), connected together mechanically with a sounding board; there is a strong coupling that results in the resonant frequencies getting "dragged down" in pitch.
Laser light is coherent while candlelight is incoherent, each photon having random phase.
However, when interacting with a medium, in combination with effects such as Brillouin scattering it may produce distorted shifts greater than the linewidth of the source.