WHY is the sun red during rise and set

It's not doppler effect. It is because of the scattering effect. The Rayleigh scattering is strong at shortwave light so you can see lots of red beams. During the sunrise and sunset, the light has to travel more in air than it does at noon.

 

it is because of scattering of light with particles in the atmosphere. During Sunrise and sunset, the angle of the incident light from the sun is different from when it is noon.... in a layman picture, you have during noon, blue light is being scattered "onto" the atmosphere while during sunset/rise, blue light is being scattered "away" and you see only the redder end of the spectrum. It is also true that light has to travel through more air (because of the angle) during sunset/rise as haiha said.