The visible white light coming from the Sun is composed from superposition of electromagnetic waves of different wavelengths ranging from the radiation that we perceive as violet, to the radiation that we recieve as red then blue, green, yellow, orange. The dust particles and droplets of water present 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 its wavelength. The gas molecules are smaller and the light behaves differently depending on its wavelength. The red light has a wavelength greater and tends to “bypass” the smaller particles without “seeing” this light, therefore, very weakly interacting with the atmosphere and its spread continuing straight along the original direction. In contrast, the blue light has a wavelength less “aware” of the presence of molecules that is reflected in all directions. Then, crossing the atmosphere, most of the majority wavelength radiation continues its trajectory straight ahead. The red light, orange and yellow are affected only minimally by the presence of air. In contrast, the blue light is diffused in all directions. In whatever direction we look, part of this light reaches our eyes. The sky therefore appears blue.

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