Twilight and Blue Sky

by: Afni K.Wardani

The basic color determination of the sky is in the wavelength scattering of sunlight by the atmospheric molecules according to the Reyleigh scattering model. The electric field of the incident sunlight oscillates the electrons in these molecules, which in turn radiate light. The overall effect is to scatter the sunlight. Light with shorter wavelengths (the red end). When the sun is near the horizo...ne the sky above an observer is therefore largerly blue. The sky more than 90 degree from the sun is less blue because it is illuminated with sunlight which must traverse a long path through the atmosphere and is therefore somewhat depleted in blue. The sky near the horizon is red or yellow because it too illuminated with light whose long traversal of the atmosphere deplets the blue.

Long before Quantum Mechanics, Lord Reyleigh (1871)analyzed scattered sunlight in terms of molecular oscillators. Using simple argument based on dimensional analysis, he correctly concluded that the intensity of scattered light was proportional to 1⁄λ^4 and therefore increases with v^4. Before this work, it was widely believed that the sky was blue because of scattering involving particle smaller than wavelength (i.e., less than λ⁄15) has been reffered to as Reyleigh Scattering. Atoms and ordinary molecules fit the bill since they are a few tenths of nanometer in diameter, where as light has a wavelength around of 500 nm. A human's blue eyes, a bluejay's feathers, he blue-tailed skins blue tail are all colored via Reyleigh Scattering.

Sunlight streaming into atmosphere from one dirrection is scattered in all directions. Without anatmosphere, the day-time  sky would be as black as the void of space, as black as the Moon Sky. When the Sun is lower than horizon, its light passes through a great thickness of air (far more so than it does at noon). with the blue-end appreciably attenuated, the reds end  yellows propagate along the line-of-sight from the Sun to produce Earth's familiar fiery sunsets.

There are other scattering models participates in the variety colors of sky phenomena, such as Mie scattering and Raman scattering. remember that Reyleigh scattering is scattering from molecules and very tiny particles (<1/10 wavelenght) predominantly. For particle sizes larger than a wavelength, Mie scattering predominates. Rayleigh scattering can be considered to be elastic scattering since the photon energies of the scattered photons is not changed. Scattering in which the scattered photons have either a higher or lower photon energy is called Raman scattering. 

References:

 Hect, Eugene.2002. 4ed Optics. Addison Wesley: St. San Francisco

Walker, Jearl.1945. The Flying Circus with Physics. Malloy Lithographing: Canada