Something is amiss in this image of a prismatic rainbow—blue appears twice!
There’s something amiss about this prismatic rainbow—can you spot it?
Image Credit: Redditor /u/sempertexas

February 18, 2016

When light is refracted through a prism, it’s split into its component colors. In this image of sunlight refracted onto an interior wall, though, something is out of the ordinary. While the colors in the photo ascend in energy from left to right, blue seems to appear twice—once on the left side of violet, and then again on the right!

So what's going on? Surprisingly, the secret is in the surface that the light is projected onto—specifically, the paint! As white paint ages, it tends to yellow. Paint manufacturers know this, so one of the things they do to combat this effect is to introduce blue fluorescent dyes into the white mixture. When ultraviolet light strikes the surface, it glows with a very faint blue light. It's ordinarily not bright enough to notice outright, but it's enough to counteract the yellowing effects of age, since blue and yellow are "opposite" colors (in some kinds of color mixing). Knowing this, it becomes perfectly clear what's happening here—we're actually seeing further into the color spectrum than we'd ordinarily be able to, as the paint's fluorescent pigments convert ultraviolet light into visible shades of blue! Many detergents and bleaches use similar technology, so if you examine your white T-shirts under a blacklight, you might see the same eerie blue glow.