We see a perfect example of the spectrum when the glamorous rainbow loops over the sky. We may see a paler echo of these rainbow colors when sunshine plays on busy garden hose or on the foamy froth that sprays up from thundering Niagara Falls. We see it again when we make sunbeams shine through a clean glass bottle of clear water. And we can make the sharpest spectrum show itself through a glass prism.
Andy, who is of course a pixie, waters his garden in the company of a hummingbird, a ground robin, a Siamese cat and a traveling rainbow. His little rainbow is made by dewy drops of water from the hose playing tricks on the sunbeams and its scientific name is the spectrum of white light.
Light is a phenomenon, a fact of nature. It is a very energetic fact, traveling along in pulsing waves at about 186,000 miles a second. In white or colorless light there are loner and shorter waves of energy, all pulsing along together. These light waves can be compared to tiny hills and dales and a wave length is the distance from one crest to the next, or from one dip to the next   both of which are the same distance.
The shortest wave length of white light is about .000038 centimeters, the longest is about .000076 centimeters. The thickness of a sheet of note paper is equal to about 130 of these long waves and about 260 of the short waves. These various wave lengths mean nothing when white light is speeding from the sun across empty space. But when they reach our atmosphere, they start to fly apart, and then we see that each wave length is a strand of color.
The short blue waves are separated by the air molecules and scattered over the sky.
At dawn and sunset, the runts light slopes through a thicker slice of air and the longer wave lengths are scattered over the sky. In a rainbow, all the wave lengths of light are unraveled in a variegated band of color. This band of color is the spectrum of white light.
A prism is a triangular shaped rod of clear glass. When a beam of white light strikes a sloping side of the prism, its different wave lengths are bent at different angles. They fan out and pass through the far side in a skein of colors. If you let it fall on a sheet of paper or a white wall, you can see clearly that the spectrum of white light is a vivid band of red, orange, yellow, green, blue, indigo and violet   with countless tones and half tones between each definite line of color.
Science uses the spectrum to identify any substance on earth or on the faraway stars. When a substance is burned, its incandescent gases can be sent through a prism and each will write its own signature on the band of colors. The gases from a speck of burning sodium will add a vivid streak to the yellow line. Helium also adds to the yellow band, but at a point nearer the blue end of the spectrum.