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Credit: Michael Theusner/Applied Optics.
This photograph shows the first-ever evidence of an elusive fourth-order rainbow. The spectacle in the image looks like a double rainbow, because it only shows the third-order (tertiary) rainbow (left), accompanied by the fourth-order (quaternary) rainbow (right). They appear on the sunward side of the sky, at approximately 40 degrees and 45 degrees, respectively, from the sun. Tertiary and quaternary rainbows can only form on the same side of the sky as the sun, unlike primary and secondary rainbows. As such, the primary and secondary rainbows are on the other side of the sky and so not shown in the photo.