Multicolored arcs in the sky catch the eye and the imagination.
Whether as semicircles of color in rain, as bright “sundogs” flanking the sun like loyal pets, or as varied arrays of circles and parts of circles around the sun, rainbows have been viewed both as symbols — of peace, diversity, and golden treasure — and as beautiful lessons in the physics of light.
Those lessons can provide insight both into rainbows and into the perceptual worlds of various animals. Although rainbows may connote serenity, the science of rainbows is all about speed — or, more precisely, differing speeds of light.
The speed of light in a vacuum, called “c” by physicists, has a special role in physics and in popular culture. The number 186,000 miles per second (or, for the more exacting, 299,792,458 meters per second, which is about 186,282 miles per second) represents both the speed limit in the known universe and the square root of the relationship between mass and energy in Einstein’s famous equation “e = mc2.”
This speed is approached only in space and in specially constructed near-vacuum equipment; light passes through familiar materials such as air, water and glass at slower speeds.
Technically sophisticated speedometers are needed to clock the rather fast speed of light in a near-vacuum, but differences in the speed of light are easily seen.
Place a straw in a glass of water, and look at it above and below the water’s surface. The apparent bend of that straw as it crosses from water to air and the focusing of a lens both result from the different speeds of light in two substances; light turns as it changes speed.
In the case of the image of a straw in water, the light increases in speed by about 33 percent as it moves into air. The “bending” of the straw may be a curiosity to us, but compensating for the effect is of vital importance to a bird catching a fish in water or to a fish catching an insect in the air.
The effect of bending due to change in speed becomes even more interesting when color is considered.
Violet light changes speed, and thus direction, about one percent more than does red light as it moves between water and air. This difference in change of speed is what causes the colors of light to spread out and form rainbows.
When sunlight from behind an observer enters raindrops, reflects from the far surfaces of the drops, and reenters the air, colors are spread to produce a classic rainbow. A double rainbow results if enough light is reflected twice inside the drops.
Ice crystals in clouds several miles above the ground act as prisms, spreading colors to produce colorful circles around the sun, the sun’s “dogs,” and, less commonly, a diverse variety of arcs and circles.
The exact pattern and degree of colorfulness of each of these phenomena depend on the shape and orientation of ice crystals in those high clouds. These displays are seen more commonly here in western Massachusetts than are the rain-produced bows.
Eye protection is important when your gaze is near the sun, so be sure to position yourself in the shadow of an object like a building to block out the sun itself when looking for these halos and arcs.
Shakespeare suggested that “to add another hue unto the rainbow” is “ridiculous and wasteful excess,” but nature has ignored the bard and done exactly that and more.
Ultraviolet and infrared light, invisible to humans, spread the rainbow beyond our senses and produce displays that look different to other animals with different color sensitivities. If people could see a rainbow as it would be seen by a bird sensitive to ultraviolet light we might find it even more beautiful.
The light spectrum shown in a rainbow is continuous, but people break it down into labelled categories — red, orange, yellow, etc. The arbitrariness of these labels is shown when people disagree about a color: The physical characteristics of light are the same whether one subjectively calls it green or blue.
Is there a pot of gold at the end of a rainbow? Does the goddess Iris traverse a rainbow to carry messages between Olympus and Earth?
I know of no definitive evidence for affirmative answers, but there are treasures of enjoyment and messages about the speed of light in every rainbow contemplated throughout one’s life. In William Wordsworth’s words,
“My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I grow old,
Or let me die!”
David Spectrum (or David Spector, as he is better known) is a former board president of the Hitchcock Center for the Environment and teaches biology at Central Connecticut State University in New Britain, Connecticut. “Coauthor” Roy G. Biv is a mnemonic for the sequence of colors in the visible rainbow: red, orange, yellow, green, blue, indigo and violet.
Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 525 South Pleasant St., Amherst, appears every other week. For more information go to www.hitchcockcenter.org, call 256-6006 or write to column@hitchcockcenter.org.