Rayleigh distance Rayleigh distance Rayleigh distance in optics is the axial distance from a radiating aperture to a point at which the path difference between the axial ray and an edge ray is λ / 4. An approximation of the Rayleigh Distance is formula_1, in which Z is the Rayleigh distance, D is the aperture of radiation, λ the wavelength. This approximation can be derived as follows. Consider a right angled triangle with sides adjacent formula_2, opposite formula_3and hypotenuse formula_4. According to Pythagorean theorem, formula_5. Rearranging, and simplifying formula_6 The constant termformula_7 can be neglected. In antenna applications, the Rayleigh distance is
Zero sound transport equation is valid in the regime where formula_36 and formula_37. In many systems, formula_38 only slowly depends on the angle between formula_39 and formula_40. If formula_29 is an angle-independent constant formula_42 with formula_43 (note that this constraint is stricter than the Pomeranchuk instability) then the wave has the form formula_44 and dispersion relation formula_45 where formula_46 is the ratio of zero sound phase velocity to Fermi velocity. If the first two Legendre components of the Landau parameter are significant, formula_47 and formula_48, the system also admits an asymmetric zero sound wave solution formula_49 (where formula_50 and formula_51 are the