Dear Fu-Kwun Hwang,
I was looking for some good explanation about rainbow physics when I found this problem and still i cannot figure out an answer to this. I've been studying some optics this year and I've been told that the primary rainbow has always the red colour on its external part. Now, as seen in picture "Rainbow1.png" the red ray is refracted downward; so the observer should perceive it as the lowest one. Could you please give me the answer to this?
Different color of sun light refracted from the same water drop are refracted at slightly different angle. The distance between the person who saw the rainbow and the water drop are normally very large. So different color light refracted from the same water drop would not enter the same person's eye. The red light and the blue light saw by the same person are sun light refracted from different water drop. Please check out rainbowcolor.jpg
If you move to different position, the rainbow you saw was not the same one. And different person will see rainbow refracted from different water drops. Everyone saw his own rainbow. The rainbow you saw is for your eyes only.