A rainbow does not have a back side. If you were to walk completely to the other side of the mist cloud that is creating the rainbow and turn around, you would not see a rainbow. You have to realize that a rainbow is not a stationary physical object. Instead, it is a pattern of light that becomes a stable image only when you look at it from the right angle. You may not have noticed it, but every time you look directly at the center of a rainbow, the sun is directly behind your head. This is the only angle at which the light pattern that constitutes the rainbow can enter your eye and therefore lead you to see it.