In the final scene of Eric Rohmer’s movie Le Rayon Vert, a young woman sits with a man she’s just met, watching the sun set over the sea. Just as it disappears below the horizon there’s a brief but unmistakable flash of green—the last fleeting moment of sunlight. The green ray, after which the movie takes its title, is said to offer a flash of clarity into one’s feelings and those of others.

Scientifically, the green ray can be explained by the way light is refracted and separated by the earth’s atmosphere as the sun approaches the horizon. The often-elusive phenomenon—people can spend their whole lives looking for it—took on a semimythical significance when it was first popularized by the publication of Jules Verne’s Le Rayon Vert in 1882, a century before Rohmer’s ...