text: how does a double rainbow occur??? i saw one the other day on my way to work...amazing...just wondered how this happens ? occasionally, a second, dimmer secondary rainbow is seen outside the primary bow. secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and appear at an angle of 50°–53°. as a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow, with blue on the outside and red on the inside. the dark area of unlit sky lying between the primary and secondary bows is called alexander's band, after alexander of aphrodisias who first described it.  hope this helps!