Why the colors change places
A rainbow is not a colored arc painted onto a cloud. It appears where raindrops send particular wavelengths of sunlight toward your eyes at particular angles. Because water bends violet light more than red light, the two colors leave a drop in different directions.
In the brighter, primary bow, sunlight reflects once inside each drop before emerging. That path puts red light along the bow’s outer edge and violet along its inner edge. For the secondary bow, the light makes a second internal reflection. The extra reflection changes the geometry of the outgoing rays: red now reaches an observer from a slightly smaller angle than violet, so the color sequence across the bow is reversed.
The dimmer bow and the dark gap
Each reflection inside a drop sends some light back into the water rather than onward toward an observer. A ray that reflects twice therefore has less light left to contribute to the secondary bow. The colors are also spread across a wider arc, making the second rainbow look broader and fainter.
Between the two bows, relatively few reflected rays reach the eye. This creates a darker region often called Alexander’s band. So the paired arcs reveal two different routes through the same drops: one route makes a bright, tightly arranged bow; the other adds a bounce, flips the colors, and leaves a dimmer arc beyond it.
