white light visible from the Sun is composed of superposition of electromagnetic waves of different wavelengths ranging from the radiation that we perceive as violet, to the radiation that appears red, passing through blue, green, yellow, orange. Dust particles and water droplets in the atmosphere are much larger than the wavelength of visible light: in this case the light is reflected in all directions equally, regardless of their wavelength. Gas molecules are smaller and the light behaves differently depending on its wavelength. Light Red has a longer wavelength and tends to "bypass" the smaller particles not "see" this light, therefore, interacts very weakly with the atmosphere and continues its straight propagation along the original direction. In contrast, the blue light has a shorter wavelength and is "aware" of the presence of the molecules from which it is reflected in all directions. Then, when crossing the atmosphere, most of the radiation of greater wavelength continues its straight course. The red, orange and yellow is affected only minimally by the presence of air. In contrast, blue light is diffused in all directions. In any direction is observed, some of this light reaches our eyes. The sky, therefore, appears blue.
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