Why is the sky blue?
In the first approximation the skies on planet Earth are deep blue. Although it seems quite obvious (because we are used to it), it is not so at all. In fact, the sky over lunar surface is rather black, and the Martian sky photographed by numerous space probes is more or less red and yellow. So why the sky is almost always blue on Earth?
To explore this matter one has to understand how the electromagnetic radiation propagates through the atmosphere. First of all, sunlight is almost fully white, though Sun is classified as a yellow star with its maximum of visual energy set to green visible wavelengths. But in fact, the spectrum emitted by the sun is a mixture of all the colors of the rainbow. So it consists of many various types of light, including very long and low-frequency red light, long orange and yellow waves, medium green rays, and finally increasingly shorter blue and purple light. This is not all. Apart from visible light our Sun also emits extremely long radio waves as well as short and very energetic UV, X and Gamma radiation. That is why scientists used to say that the spectrum of solar radiation is continuous, and some of its energy travels in short waves while other light moves in long, lazy waves. The secret key to the whole mystery of the blue sky is that blue, more energetic waves of light are shorter than the red ones.
In the presence of an absolute vacuum and without any gravitational distortions every ray of light travels in a straight line. But it gets more interesting when something happens to them along the way. When light beam hits an obstacle, for example in the form of a particle suspended in the Earth’s atmosphere, its course may change in one of three possible ways. Light can be reflected (like in a mirror), bent, or just scattered. Most of the sunlight hitting the atmosphere is scattered in all directions by the gases present in the air, like oxygen, nitrogen, carbon dioxide and some additional molecules. And the blue light is better scattered on the tiny molecules of the air, more than other wavelengths, because it used to travel as shorter waves that are more likely to distort and to change their paths. This is the general reason why we see the blue sky over Earth.
This effect is called the Tyndall effect, but it is more commonly known as Rayleigh scattering. Rayleigh was the one who studied the problem in more detail and formulated its mathematical description. In this effect the total amount of light scattered is inversely proportional to the fourth power of wavelength, if only the molecules considered here are tiny enough. In the average Earth atmosphere the blue light is scattered more than red light by a factor of (700/400)^4, which is close to ten. But in truth the things are not so simple. Today we ‘know’ that the color of the sky results directly from the scaterring on small droplets of water vapor. This idea is still very popular, but incorrect. Light has to be scattered on smaller molecules of oxygen and nitrogen. Actually the one who proved it and showed that this result is in a good agreement with observations was famous Albert Einstein. He also deduced that phenomenon of light scattering is not purely mechanical, but rather electromagnetic, or even quantum effect in which the scattered photons of light change their energies. This process, if considered at the molecular level, involves exciting so-called vibrational mode of the molecules. It happens when, for example, oxygen is giving a lower scattered photon additional energy, or by the scattering of previously excited vibrational state of a molecule which then transfers its vibrational energy to the incident photon. To say it more briefly, some atmospheric particles scatter light due to the presence of the electromagnetic field of the light waves that induces electric dipole moments in these molecules.
So why sometimes the western or eastern sky is glowing with many different colors? Because of the temporary change in the atmospheric composition. If, for example, there is a lot of water vapor in the air, the average molecules of the air are bigger and can effectively disperse also the red light. Moreover, when the sun is low in the sky, its light has to pass through a thicker mass of air before reaching the observer. Then the numerous air particles can not only scatter once, but also re-scatter the light many times. The final result of this process is the total mix of many various wavelengths and directions of rays, so the effective lighting seems to be more white then blue. If the air is additionally polluted with any small particles, the sunset seems more red. It happens very often over the sea or within large industrial areas.
A bit different, however, is the situation in the case of other planets. Scientists believe that the reddish color of the Martian sky results from the presence of red iron-rich dusts thrown up in the dust storms. The exact color depends on actual weather conditions and it seems more blue when Martian atmosphere is relatively calm. And what about our Moon? This celestial body has almost no atmosphere, so the light is not scattered at all. That is why lunar sky seems almost black and its shadows are extremely sharp.
Author: Elzbieta Kuligowska