Linda Liberatore, age 14, of East Boston, Mass., for her question:
What does a spectroscope do?
How can a scientist name the gasps of the sun? He uses an instrument called the spectroscope. How can a chemist name the elements in a bit of metal? The spectroscope may tell him the answer. This instrument is a detective machine which forces an element to sign its name. And the signature is written in color ‑ in one or more of the colors of the rainbow. When burned in a colorless flame, an element gives off its own color and that color belongs to this one element only.
The word spectroscope means, of coursed the spectrum viewer. The spectrum is the rainbows the band of colored rays of which ordinary light is made. We say there are seven colors of the spectrum ‑ red, orange, yellows green, blues indigo and violet. Actually, these bands of color blend from one to another with countless different tones between.
We do not see the spectrum in ordinary light. Here the rainbow colors are blended together. They are blended together and hidden as the light whips across space at about 186,000 miles a second. However, this skein of hidden colors can be separated. This happens in the rainbow and when white light strikes a glass prism. The prism is an elongated pyramid of glass. Then white light strikes its sloping sides it is forced to bend. And the different colors must bend, And the different colors must bond at different angles. This unravels the skein of white light and forces it to show its rainbow colors.
A prism is one of the important items in the tube of the spectroscope. The element to be tested must be burned in a flame which has no color of its own to confuse the signature. As the element burns, it gives off an incandescent gas. The color of this gas is the telltale signature.
A few elements have a very bold John Hancock type signature. The element sodium, for instance, gives off a vivid yellow color ‑ once seen, never forgotten. A skilled chemist may recognize the spectrum signature of sodium without the help of a spectroscope. However, if he is a thorough worker, he will want to check the color through his spectroscope.
The color of the incandescent gas passes through a prism and system of lenses all inside the spectroscope tube. The spectroscope reveals exactly where each color signature fits into the spectrum. Most elements produce two lines across the spectrum. The vivid yellow of sodium is produced by two lines close together in the yellow section of the spectrum. Helium produces a yellow line, close enough to sodium to fool the eye. But the spectroscope shows that the yellow line of helium is closer to the violet end of the spectrum. A chemist could also be fooled by the spectra of the two elements lithium and rubidium. Both give off a rich red color, almost impossible for the eye to tell apart. But the spectroscope cannot be fooled. The two reds show up as bands in different parts of the spectrum.
Only a tiny fragment of material is necessary for the spectroscope signature. But the substance must be turned to a burning gas. Substance composed of different elements will reveal the colored signatures of each. Since the sun and stars are composed of incandescent gasps, the spectroscope is used to tell what elements are present. The element helium was discovered in the sun by this method before it was discovered on earth.