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Why are sunsets red?
The intense red and orange hues of the sky at sunset are mainly caused by scattering of sunlight by dust particles, soot particles, other solid aerosols, and liquid aerosols in the Earth's atmosphere. These enhanced red and orange colors at sunset are mathematically explained by the Mie theory or the discrete dipole approximation. (We won't go into that)
Sunlight, called white light, is made up of different colors of light, each having a different wavelength. During a sunset, more red light is scattered toward you because of the aerosols, compared to the amount of blue or green light.
At sunset, sunlight passes through a much longer path of the lower atmosphere than when the sun is overhead, so the effect of the aerosols becomes much stronger. So, you end up seeing more red light that any of the other colors of light, and the sky appears red.