In a double rainbow, the colors of the outer arc are reversed.

A primary rainbow is formed when sunlight enters a water droplet, reflects once off the back surface, and exits. This process refracts the light into a spectrum with red on the outside at an angle of approximately 42 degrees. In contrast, a secondary rainbow occurs when light reflects twice inside the droplet before escaping.This second internal reflection causes the light to exit at a steeper angle of about 51 degrees, which is why the secondary arc appears higher in the sky. Because light is lost with each reflection, the secondary rainbow is significantly fainter than the primary one. The double reflection also inverts the color order, placing violet on the outer edge and red on the inner edge.The space between the two arcs is known as Alexander's Dark Band, named after the Greek philosopher Alexander of Aphrodisias who first described it in 200 AD. This area appears darker because light is reflected into the primary and secondary bows but not into the region between them. This phenomenon is a perfect demonstration of the law of reflection and the refractive index of water.