We expect the last colour of the sunset to be on the red end of the visible spectrum because of the dispersion of light. Due to the particles present in the atmosphere, the light rays undergo scattering and the light of shorter wavelengths like blue gets dissipated but light rays of longer wavelengths travel long distances without getting dissipated.
Dispersion of light is the phenomenon in which the light from the sun is split into its constituent colours which are Violet, Indigo, Blue, Green, Yellow, Orange, and Red when it passes through a prism or due to the particles present in the earth's atmosphere.
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