Rayleigh scattering
In 1869, John Tyndall observed faint blue-tinted light scattering off nanoscopic particulates in purified air. He conjectured that a similar process gave the sky its blue hue but could not explain why blue light was preferred over red. Atmospheric dust failed to account for the intensity of the color he saw. Lord Rayleigh published two papers on the color and polarization of skylight in 1871 to quantify Tyndall's effect. These papers described tiny particulate volumes and refractive indices within water droplets. By 1881, James Clerk Maxwell's 1865 proof of the electromagnetic nature of light allowed Rayleigh to show his equations followed from electromagnetism. In 1899, he demonstrated that these principles applied to individual molecules. This final paper established the basic scientific model for the color of the sky.
The oscillating electric field of a light wave acts on charges within a particle smaller than the wavelength. Charges move at the same frequency as the incoming wave. The particle becomes a small radiating dipole whose radiation we see as scattered light. Particles may be individual atoms or molecules found in gases. It occurs when light travels through transparent solids and liquids. Scattering applies to particles with a size much smaller than the wavelength of the radiation. Objects with a dimensionless parameter x much greater than one act as geometric shapes scattering light according to their projected area. At an intermediate value near one, interference effects develop through phase variations over the object's surface. Rayleigh scattering applies when the scattering particle is very small with a radius less than one-tenth of the wavelength.
Blue light wavelengths scatter more strongly than longer red wavelengths due to the inverse fourth power dependence. Shorter blue wavelengths are scattered out of the direct path of sunlight reaching the ground. The diffuse sky seen in daytime appears blue because of this strong wavelength dependence. At twilight, the low Sun shows a yellowish to reddish hue since blue light has been removed. Some scattering comes from sulfate particles following large Plinian eruptions. Years after such events, the blue cast of the sky is notably brightened by persistent stratospheric gas loads. J.M.W. Turner painted vivid red colors possibly influenced by the eruption of Mount Tambora during his lifetime. In locations with little light pollution, the moonlit night sky also appears blue because moonlight is reflected sunlight.
Rayleigh scattering serves as an important mechanism for wave scattering in amorphous solids like glass. It causes acoustic wave damping and phonon damping in glasses at low or not too high temperatures. Higher temperatures obscure the Rayleigh-type regime with anharmonic damping that becomes increasingly important as heat rises. This temperature-dependent shift changes how sound travels through granular matter. The damping follows a lambda negative two dependence on wavelength rather than the standard lambda negative four pattern found in optics. Scientists study these effects to understand material behavior under thermal stress. Glass remains a disordered material where microscopic density variations dictate energy loss patterns.
Silica fibers are glasses containing microscopic variations of density and refractive index. These fluctuations give rise to energy losses due to scattered light signals traveling through the medium. A fictive temperature represents the point at which density fluctuations freeze into the material structure. The Boltzmann constant and isothermal compressibility factor into calculations for signal attenuation. Refraction indices and photoelastic coefficients determine the magnitude of this optical scattering effect. Engineers must account for these losses when designing long-distance communication networks. Nanoporous materials exhibit strong contrast between pores and solid parts resulting in very strong scattering. Light completely changes direction every five micrometers on average within sintered alumina powder structures.
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Common questions
When did John Tyndall observe faint blue-tinted light scattering off nanoscopic particulates in purified air?
John Tyndall observed faint blue-tinted light scattering off nanoscopic particulates in purified air in 1869. He conjectured that a similar process gave the sky its blue hue but could not explain why blue light was preferred over red.
What year did Lord Rayleigh publish papers on the color and polarization of skylight to quantify Tyndall's effect?
Lord Rayleigh published two papers on the color and polarization of skylight in 1871 to quantify Tyndall's effect. These papers described tiny particulate volumes and refractive indices within water droplets.
Why do shorter blue wavelengths scatter more strongly than longer red wavelengths according to Rayleigh scattering principles?
Blue light wavelengths scatter more strongly than longer red wavelengths due to the inverse fourth power dependence. Shorter blue wavelengths are scattered out of the direct path of sunlight reaching the ground, causing the diffuse sky seen in daytime to appear blue because of this strong wavelength dependence.
How does temperature affect acoustic wave damping and phonon damping in glasses at low or not too high temperatures?
Rayleigh scattering causes acoustic wave damping and phonon damping in glasses at low or not too high temperatures. Higher temperatures obscure the Rayleigh-type regime with anharmonic damping that becomes increasingly important as heat rises.
When did James Clerk Maxwell prove the electromagnetic nature of light allowing Rayleigh to show his equations followed from electromagnetism?
James Clerk Maxwell proved the electromagnetic nature of light in 1865 which allowed Rayleigh to show his equations followed from electromagnetism by 1881. In 1899 he demonstrated that these principles applied to individual molecules establishing the basic scientific model for the color of the sky.