Few people have never seen a rainbow, the beautiful arc of color in the sky after a rain
shower. Amazingly, no two people see the same rainbow because it is a reflection that
changes according to one’s perception. If a viewer moves toward the phenomenon, it moves
back at an identical pace. Because it is made entirely of light, rainbows can disappear
as quickly as they appear.
It was René Descartes of the 17th century who first figured out the precise optical
geometry that forms a rainbow. To see it, the onlooker must stand between sunshine and
rain with his or her back to the sun. When sunlight shines past the observer and
penetrates the raindrop, it is refracted, or bent. After the light crosses the drop to
the far inner surface, the light is reflected back toward the spectator. As the reflected
light comes forth from the raindrop on the same side it entered, it is refracted again
and tilted at a 42 degree angle to its original path from the sun. The tilt is what
gives the rainbow great dimensions: it can reach halfway to the highest point above the
person and spread across almost half the horizon in front of them.
The light, other than being bent and tilted, also changes from a single white beam into
an array of colors. A generation later, Isaac Newton found the reason why. Sunlight is
composed of many different wavelengths of light, each with its own characteristic angle
of refraction. The highest of bands is red, then orange, green, blue, indigo, and violet.
The rainbow’s colors appear most clearly when the raindrops are fairly large. Colors
soften and blur when drops are too small or too large.
Aristotle was the man who first described the rainbow’s true form 2,300 years ago. It is
in fact a circle, whose center is in alignment with the viewer and the sun. However, the
full circle can only be seen in rare instances, like in the mist from a waterfall, from
an airplane, or on a mountaintop. It is the earth’s shadow itself that blocks sunlight
from completing the circle for surface-bound observers.