There are at least two conditions, each known as the "Rayleigh criterion." The first states that Couette flow will be unstable if
where is the angular velocity and and are the inner and outer radii of cylinders.
The Rayleigh criterion for interference is satisfied when the center of the Airy disk of one image is superimposed on the first minimum of the other. This occurs when the combined irradiance of both fringes at the center of the resultant broad fringe is times the maximum irradiance. For lenses,
For mirrors,
Airy Disk, Couette Flow, Lens, Mirror
