Check out my facebook post of a double rainbow photo. These are often called first and second order rainbows. A third order rainbow can only be seen by looking into the sun…rare and I don’t advise that! The first order or primary rainbow is most common, and has the red colors on the top and the blues/violets on the bottom of the rainbow. The color split is caused by a prism-like separation of refracted sunlight into its colors ordered by the wavelength of each color, the reds are longest and hence bent most, the blues/violets shortest and hence are bent less. The refraction caused by light moving through air and then water. Note that very often inside the “primary” rainbow the sky is brighter or more well lit than outside the rainbow. This is because rain drops are not perfect spheres and some light is preferentially reflected toward the inside portion of the rainbow.
When a second order rainbow is visible it is nearly always fainter and the color order is reversed, red is on the bottom side! This is because this second bow is cause by one additional reflection of light within the raindrops, that reflection dims the bow but like a single mirror reflection reverses the image (hence the colors). Details on exactly how reflection and refraction of light through raindrops occurs gets complicated but these details are not necessary to provide you this fundamental understanding. Of course to see a rainbow turn your back to the sun and look toward the rain. The extent of the rainbow will depend on the arial extend of rain and the sun angle. Near sunset and looking into a large rain area, nearly half-circle of rainbow may be seen.
Dr. Steve Lyons 