Awesome question. The most basic answer is that light is refracted by particles in the atmosphere and the red end of the spectrum is what is visible. read more
The explanation lies in-between. Light from the sun consists of three kinds of radiation – ultra-violet rays, visible light and infra-red rays. read more
During sunset hours, the light passing through our atmosphere to our eyes tends to be most concentrated with red and orange frequencies of light. For this reason, the sunsets have a reddish-orange hue. The effect of a red sunset becomes more pronounced if the atmosphere contains more and more particles. read more