The redness in the atmosphere at sunrise and sunset is due to

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Scattering of light is the phenomenon in which a light ray is redirected in all other directions after passing through particles with dimensions comparable to the wavelength of the light used.

White light has seven colours: violet, indigo, blue, green, yellow, orange, and red. The wavelength of red light is the longest of the seven constituent colours.

During the sunset and sunrise hours, the light passing through our atmosphere to our eyes is most concentrated with red light because it has the longest wavelength of all and has almost no scattering. That's why it appears red.