But somewhere in humans' evolutionary history, that photoreceptor shifted more toward detecting violet than ultraviolet wavelengths. Perhaps we couldn't afford the damage to our eyes because we ...

"Why are there so many songs about rainbows?" sings Kermit the Frog in a song (about rainbows) called "Rainbow Connection" from 1979's "The Muppet Movie." And it's not just songs that feature rainbows ...