The lower the sun is, the more of our earth’s atmosphere scatters the light, which travels through. But as we know that sunlight is a mixture of seven colors. Normally, this mixture of light appears as white to our eyes. And the atmosphere has molecules of air, dust, water vapor, and other impurities in it. As the light passes through them, different colors are scattered by these particles. Now, it so happens our atmosphere scatters out violet, blue, and green light more than it does the reds and yellows. So when the sun is low, this scattering leaves more reds and yellows for us to see and we have a reddish sunset.