posted January 09, 2012 07:27 PM            

The Rainbow Bridge

In Norse mythology, the rainbow is the road between the worlds of gods and men. It is a substantial thing: Horses are ridden over it, their hooves clattering; Heimdall, the gods’ watchman, even built his house upon it. At Ragnarok, the "doom of the gods," when the fire ogres and frost giants storm up it to destroy the gods’ home, the rainbow bridge will break.To think of a rainbow as the "bridge to the divine" is common throughout world mythology. Raymond L. Lee Jr. and Alistair B. Fraser, authors of The Rainbow Bridge: Rainbows in Art, Myth, and Science, found the concept in Zulu, Navaho, Hawaiian, Japanese, Cambodian, Greek, Australian Aborigine, Chumash, and Hopi myths, as well as the Norse. "The apparently huge size of a rainbow seen in distant rain," they write, "makes the notion of a bridge to heaven’s heights easily understandable."

Turning the myth into a metaphor, in The Rainbow Bridge Lee, an adjunct professor at the U.S. Naval Academy, and Fraser, a professor of meteorology at Penn State, show how the rainbow can also bridge science and art — and how it often fails to do so.

Beginning in the 17th century, they write, "artists increasingly portrayed [the rainbow] as incidental visual filler, an impressive landscape component, or an object of scientific curiosity." They didn’t, however, get the optics right. They set the rainbow at incorrect angles to the sun, or colored it in impossible hues. Yet as rainbow science improved, artists’ representations generally did not. Much of what we recognize as a rainbow — in art or advertising — could never appear in nature (or, as Lee and Fraser put it, describing one modern ad, "Reality is pretty clearly on holiday here").

The early artists were not really at fault. The full explanation of how a rainbow "works" wasn’t available until James Clerk Maxwell presented his theory of optics in 1862. Lee and Fraser trace the history of our understanding of rainbows, asserting that "the rainbow bridge links the sciences to their past." The Rainbow Bridge takes us from Aristotle through Avicenna to Theodoric of Freiburg, who "plausibly (although not quite correctly)" used refraction and reflection within a raindrop to explain the rainbow’s colors; then through the "farfetched model" of the greatest geometer of the 15th century and the "maunderings" of the father of electricity in the 16th; to "Kepler’s rainbow muddle," Descartes "groundbreaking essay," and Newton’s "truly satisfactory account of the rainbow’s colors"; until finally we reach Maxwell’s full explanation.

"Optically, the rainbow is just a distorted image of the sun," Lee and Fraser write in the appendix, "A Field Guide to the Rainbow." "Think of the drops as imperfect one-way mirrors," the authors suggest. "The light forming the primary rainbow is that refracted on entering the drop, reflected at his rear, and refracted a second time on exiting." When light passes through the air-water boundary, it is split into its spectrum, with the blue light bent more than the red, giving the rainbow its colors. Its bow shape is also a function of optics: "Many drops acting in concert cause the rainbow, and all of these must be at the same angle from the sun (that is, the same angle from the antisolar point). Thus at any instant only those drops before you that are on a 42-degree circle centered about the antisolar point can send you the concentrated rainbow light. These drops may be at any distance, but they must be on the 42-degree circle. Put another way, the rainbow is a mosaic of light sent to you by many raindrops as they fall through the surface of the imaginary cone whose tip is at your eye and whose radius is 42 degrees."

ow, with the rainbow’s optical puzzle solved, the discrepancy between real rainbows and artists’ renditions can no longer be called an accident. "Indulging in fantasy for its own sake is one of life’s great pleasures, and rainbow fantasies are no exception," Lee and Fraser write. "However, being unable to distinguish between rainbow fact and fantasy is no asset." From some of the many illustrations presented in The Rainbow Bridge, from postcards and advertisements to realistic landscapes, it seems that ability is not widespread. "Our goal here is not to cast artists in the role of scientists," the authors continue, "but rather to emphasize the shared problems of observation that the two groups face — and how they often arrive at very different answers."

The rainbow can never be seen obliquely, for instance, as many artists have painted it, because it has no physical form. It is a mirage, "a mosaic image of sunlit rain," centered on a point directly opposite the sun (which also means the sun can never be seen peeking out from behind a rainbow, as some advertisements have it). Because the relationship of sun to rainbow is fixed, all shadows in a painting must converge on the rainbow’s center. To see a rainbow, you must stand with the sun directly behind you and the rain before you. The rainbow will appear around the head of your own shadow.

Which brings us back to mythology: The connection between observer and rainbow, Lee and Fraser note, "has several interesting implications. In particular, it means that the rainbow is a terribly personal phenomenon. No two people see exactly the same rainbow because each sees it around the head of his or her own shadow." Which also means that you can’t run away from it or catch it — although people try, including Fraser once when driving through Canada. "Although he traveled down the highway quite rapidly," the authors write, "his rainbow kept pace with his shadow as both raced through the Saskatchewan wheat fields."

That the rainbow does so "probably explains many cultures’ fear" of it. While Westerners may think first of finding the pot of gold at the rainbow’s end, "in a threatening world something that cannot be outrun is an unwelcome sight." As Lee and Fraser write, "You may not have thought about the dangers of pointing at the rainbow, but rest assured that much of humanity has. In places as widely separated as Hungary, China, Mexico, and Gabon, pointing (or even looking) at a rainbow is a foolhardy act." The consequences include "getting jaundice, losing an eye, being struck by lightning, or simply disappearing" — perhaps being taken over the rainbow bridge to the home of the gods.

—Nancy Marie Brown http://rps.psu.edu/0109/rainbow.html