Andrew Kenney, age 12, of Milwaukee, Wisconsin, for his question:
Are the rainbow colors always in the same order?
The rainbow, as we know, is the spectrum of white light. There is a built in reason why the banded colors are always in the same order. However, sometimes this same series stands on its head. When one glimmering rainbow loops over the sky, the red bands are on top of the arch and the blue bands on the underside. When a paler secondary bow appears outside the primary one, the same series of colored bands is reversed.
Ordinary daylight is merely a small slice of the radiant electromagnetic energy that pours out from the sun at 186,000 miles per second. Actually it is a blend of energies pulsing along on different wavelengths. The angstrom unit is used to measure their infinitesimal distances from crest to crest. One centimeter is slightly longer than half an inch and equal to 100 million angstrom units. The wavelengths of ordinary white light range from about 3,800 to 7,800 angstroms, all blended together in invisible radiant energy.
When a beam of white light strikes through the glassy sides of a prism, it is bent, or refracted. Its different wave lengths are refracted at different angles and separated from the invisible blend. They are sorted according to wave length into a series of bands that become visible as the rainbow colors of the spectrum. Actually the colors are the short, medium and longer wave lengths arranged in precise order.
The shortest waves in the 3,800 to 4,400 angstrom range reveal themselves as violet bands of color. Next in order are the blue rays in the 4,400 to 4,900 range. The green rays must appear next in the orderly alignment because their wave lengths are in the next range from 4,900 to 5,600 angstroms. The yellows in the 5,600 to 6,000 range are next, then the orange rays with wavelengths between about 6,000 and 6,300 angstroms. The longest rays create the red band at the top of the spectrum. The order is precise because different wavelengths are refracted at precise angles.
A rainbow performs the same trick with sunbeams and falling raindrops that act like tiny glass prisms. It appears when the sun shines in the east or west and a raining cloud is in the opposite part of the sky. The raindrops refract the wavelengths of sun¬light at different angles and separates them to reveal their colors. The shortest waves are refracted at the sharpest angles and separated into the blue bands at the bottom of the bow. The longer and still longer waves are refracted at wider and still wider angles. Each series of angles reveals the next band of color.
As a rule, we classify seven bands of rainbow colors. Actually, the multitude of wavelengths merges from one band to the next. If our eyes were sharper, we could detect uncountable shades from dark indigo, through violets and blues, greens and yellows, oranges and reds.
In a single rainbow, the blues are at the bottom and the reds at the top. But the same laws of light and refraction can repeat themselves and display a larger secondary bow outside the primary bow. Its paler colors are reflected from the rain¬drops twice and the sequence reverses to show the reds below and the blues above. Three, four or five rainbows are possible, so are rainbows of mainly reds or reds and greens.