Spectroscopy
Splitting light into its colours to read a chemical fingerprint: each element emits and absorbs light at its own wavelengths.
Spectroscopy is the study of light split into its component colours. When a hot gas glows, its atoms give out light only at particular wavelengths, seen as bright lines in a spectrum. When light passes through a cooler gas, the same atoms absorb those same wavelengths, leaving dark lines. Each element has its own pattern, as distinctive as a barcode.
The yellow of sodium street lamps, and of salt sprinkled into a flame, comes from a pair of sodium lines at about 589 nanometres. See those lines through a spectroscope and you know sodium is present, whether the sample is in a test tube or in a star.
In 1802 William Hyde Wollaston noticed dark gaps in the spectrum of sunlight, and in 1814 Joseph von Fraunhofer mapped more than 570 of them. Around 1859 Gustav Kirchhoff and Robert Bunsen realised that many of these dark lines match the bright lines given out by elements heated in the laboratory. The dark lines are made by atoms in the Sun's atmosphere absorbing light from below. For the first time, scientists could tell what a distant object was made of without touching it.
Because the pattern does not depend on distance, the same method works for a fire in a nearby city, a star or a galaxy. Small shifts in the lines also reveal motion, through the Doppler effect.
Coming up in future stories
- They Read a Fire in the Next City 1859 · Heidelberg, Germany