The perception of a rainbow depends on the observer's eyes lying in line with the different hues the white light of the Sun is turned into by raindrops. The higher these raindrops are in a rainbow, the redder they look to the observer; and the lower, the bluer.
Imagine a human eye in the lower left corner of the above illustration and try to answer the following questions. When can a raindrop be seen? When can a raindrop not be seen? What colour will a raindrop look like to the observer when it is risen or lowered above the ground (by, for example, dragging it up and down)? Why are secondary rainbows always higher than primary rainbows? Why are secondary rainbows always dimmer than primary rainbows?