An Etymological Dictionary of Astronomy and Astrophysics
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A c.g.s. unit of light intensity used in astronomy and physics to measure the brightness of the night sky, auroras, etc. One rayleigh (R) represents the light intensity of one million photons of light emitted in all directions per square centimeter of receiver per second; or, in SI units, 795.775 x 10⁶ photons per square meter per steradian (m^-2·sr^-1). A dark night sky has a light intensity of roughly 250 R.

In honor of the English mathematician and physicist Lord Rayleigh (1842-1919), surname of John William Strutt, Third Baron Rayleigh, whose research ranged over several fields of physics.

1) A straight line that connects the points corresponding to the initial and final states on a graph of pressure versus specific volume for a substance subjected to a → shock wave. The slope of the Rayleigh line is proportional to the square of shock speed. Steeper Rayleigh lines correspond to higher shock speeds. See also → Hugoniot curve.
2) In → Raman scattering, the spectral line in scattered radiation which has the same frequency as the corresponding incident monochromatic radiation.

→ rayleigh; → line.

The ratio of the buoyancy force to the viscous force in a medium. This dimensionless number is used to estimate when convection commences in a fluid. It depends on the density and depth of the fluid, the coefficient of thermal expansion, the gravitational field, the temperature gradient, the thermal diffusivity, and the kinematic viscosity. Convection usually starts when Ra is 1000 or more, while heat transfer is entirely by conduction when Ra is less than 10.

→ rayleigh; → number.

The scattering of light by → particles of size small compared with the → wavelength of light. The intensity of the light scattered by unit volume of the medium at an angle θ to the direction of propagation of the incident light is: I_θ = 8 π⁴α² N I₀ (1 + cos²θ)/(R²λ⁴), where α is the → molecular polarizability, N is the number of scattering molecules, I₀ is intensity of the incident light, λ is the wavelength, and R is the distance from the scatterer. The fourth power dependence on wavelength means that blue light is much more strongly scattered than red light from a medium containing very fine particles. The air molecules, mostly → nitrogen (78%) and → oxygen (21%) are some 1,000 times larger than → visible light wavelengths. This accounts for the bluish appearance of smoke and of clear sky when the observation is not along the direction of illumination. The setting Sun, seen through a considerable thickness of atmosphere appears reddish because long wave radiation predominates in the transmitted light.
Historically, John Tyndall first discovered this phenomenon in 1859 (→ Tyndall effect), but Lord Rayleigh studied it in more detail in 1871.

→ rayleigh; → scattering.

A criterion for the instability of a basic swirling flow with an arbitrary dependence of angular velocity Ω(r) on the distance r from the axis of rotation. This states that in → inviscid fluids: Ω(r) < 0 for instability, where Ω = (1/r³) (d/dr)(r⁴Ω⁴).

→ Rayleigh; → criterion.

A classical law approximately describing the intensity of radiation emitted by a → blackbody. It states that this intensity is proportional to the temperature divided by the fourth power of the wavelength (8πkT/λ⁴). The Rayleigh-Jeans law is a good approximation to the experimentally verified Planck radiation formula only at long wavelengths. At short wavelengths it runs into a paradox named the → ultraviolet catastrophe.

→ Rayleigh; → Jeans; → law.

The part of → electromagnetic spectrum approximated by the → Rayleigh-Jeans law.

→ Rayleigh; → Jeans; → spectrum.

A type of hydrodynamical instability between two fluids of different densities, which occurs when the heavy fluid lies above the lighter fluid in a gravitational field. More generally a material interface is said to be Rayleigh-Taylor unstable whenever the fluid acceleration has an opposite direction to the density gradient.

→ rayleigh; → Taylor number; → instability.