English: Schematic representation of the frequency shift of the counterpropagating waves in a ring laser interferometer.

The ring laser interferometer is rotating counterclockwise. Laserlight propagates in both directions in the ring-shaped laser cavity. The clockwise propagating laserlight is depicted as traveling a shorter distance than the total length of the laser cavity. The counterclockwise propagating laserlight is depicted as traveling a longer distance than the total length of the laser cavity. The lasing process that generates the laserlight is a process of resonance. In the absence of rotation the laserlight has the same frequency in both directions. When the ring laser setup is rotating there is a splitting of the frequencies. It turns out that the counterpropagating light must always go through the same amount of cycles as the co-propagating light. The wavelength, hence the frequency, shifts in such a way that for both directions of propagation the number of cycles is the same.

For clarity, in the diagram the counterpropagating and co-propagating laserlight have been separated. In an actual ring laser interferometer this is not the case of course