Rayleigh scattering

Rayleigh scattering (named after Lord Rayleigh) is the scattering of light by particles much smaller than the wavelength of the light. It occurs when light travels in transparent solids and liquids, but is most prominently seen in gases. Rayleigh scattering of sunlight from particles in the atmosphere is an early approximation of the reason for why the light from the sky is blue.

So why is the sky blue instead of violet?

Because of the strong wavelength dependence (inverse fourth power) of light scattering according to Raleigh's Law, wouldn't one expect to see a violet sky rather than a blue one? There is a simple physiological explanation for this apparent conundrum. It turns out that the human eye's high resolution color-detection system is made of proteins and chromophores (which together make up photoreceptor cells or "Cone" structures in the eye's fovea) that are sensitive to different wavelengths in the visible spectrum (400nm-700nm). In fact, there are three major protein-chromophore sensors that have peak sensitivities to Red (580nm), Green (540nm), and Blue (450nm) light.

Related Topics:
Physiological - Protein - Chromophore - Photoreceptor cell - Cone - Fovea - Visible spectrum

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When one experimentally plots the sensitivity curves for the Rho, Gamma, and Beta color sensors, three "bell-curve" distributions with peaks at each of these wavelengths is seen to overlap one another and cover the visible spectrum. We depend on this RGB color sensing to detect the entire spectrum of visible light. However, when one looks at the bell-curve sensitivity of the Beta (blue) color sensor, there is a narrow detection-band with a rapid fall-off in sensitivity around 450nm. This means that our eyes are many times less sensitive to Violet light (400nm) than to Blue light!

Related Topics:
Bell-curve - RGB

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But not to worry, birds who have up to four color detecting sensors have one with a peak sensitivity at 370nm which is in the UV spectrum! Ever wonder what an ultraviolet sky looks like?

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