The first step towards correctly exploring the color of the Sky was taken by John Tyndall in 1859. He discovered that when light passes through a clear fluid, holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red. This is called the Tyndall Effect, but it is more commonly known to physicists as ;Raleigh Scattering; after Lord Raleigh studied it in more detail a few yeas later. He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles.
It follows that blue light is scattered more than red light by a faction of (720/400)4 »10. The eye has three types of color receptors, or cones, in our retina. They are called red, blue and green receptors because they respond most strongly to light at these wavelengths. As they are stimulated in different proportions, our visual system constructs the colors we see.
It is known that the blue receptors respond to light over a broad range of high frequencies, green receptors respond to medium frequencies and the red receptors respond to low frequencies. The ranges of sensitivity of the receptors overlap considerably, but they have their maximum sensitivities at different frequencies. When we look up at the sky, the red cones respond to the small amount of scattered red light and also less strongly to orange & yellow wavelength.
The green cones respond to yellow and the more strongly scattered green & green-blue wavelengths. The blue cones are stimulated by colors near blue-wavelengths, which are very strongly scattered. If there were no indigo and violet in the spectrum of sunlight, the sky would appear blue with a slight green tinge. However, the most strongly scattered indigo and violet wavelengths stimulate the red cones slightly as well as the blue, which is why these colors appear blue with an added red tinge. The net effect is that the red and green cones are stimulated about equally by the light from the sky while the blue is stimulated more strongly. This combination accounts for the pale sky blue color.
Tyndall and Raleigh thought that the blue color of the sky was due to small particles of dust and droplets of water vapor in the atmosphere. However, if this were to be true, there would be more variation of sky color with humidity or large conditions than was observed. Therefore, they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for scattering. Scattering occurs when the atoms of a transparent material are not smoothly distributed over distances greater than the length of a light wave, but are bunched up into lumps of molecules or particles. The sky is bright because molecules and particles in the air scatter sunlight. Light with higher frequencies and shorter wavelengths is scattered more than light with lower frequencies and longer wavelengths.
The atmosphere scatters violet light the most, but human eyes do not see this color, or frequency, well. The eye responds well to blue, though, which is the next most scattered color. Sunsets look red because when the sun is at the horizon, sunlight has to travel through a longer distance of atmosphere to reach the eye, than the distance from the sun during the day. The thick layers of air dust and hazes scatters away much of the blue.
Einstein finally settled the case in 1911 who calculated the detailed formula for the scattering of light from molecules which was found to be in agreement with the experiment. The molecules are able to scatter light because the electromagnetic field of the light wave induces an electric dipole moment. Molecules are usually electrically neutral, but they are made of changed objects: their atoms consist of negative charged electrons and positively charged nucleus. If there is an electric field at the position of an atom, the nucleus will move a short distance in the direction of the field of the electrons will move the other way and the atom will become dipole.
The positive and negative charge will be centered around different places, If the dipole is made to oscillate, i.e., if the positive and negative charge wiggle back and forth, out of place with each other, then the molecule can produce electromagnetic radiation of its own. This is how air molecules scatter light. The scattered light is biased towards the high frequencies and as explained earlier, the blue receptors of the retina make the sky appear blue. Likewise, when earth is seen from space, earth looks blue.