text: Between these two limits is a point at which the dispersion caused by gravity cancels out the dispersion due to the capillary effect.  At a certain wavelength, the group velocity equals the phase velocity, and there is no dispersion. At precisely this same wavelength, the phase velocity of gravity–capillary waves as a function of wavelength (or wave number) has a minimum. Waves with wavelengths much smaller than this critical wavelength  are dominated by surface tension, and much above by gravity. The value of this wavelength and the associated minimum phase speed  are:
  For the air–water interface,  is found to be , and  is .
  If one drops a small stone or droplet into liquid, the waves then propagate outside an expanding circle of fluid at rest; this circle is a caustic which corresponds to the minimal group velocity.