Reality → Energy → Light → Spectroscopy
Spectral analysis of light and other electromagnetic radiation is at the base of much of our knowledge about atoms and the universe. Dispersion of sunlight into the colors of a rainbow as caused by a prism was long regarded as a demonstration of the wave nature of light [1] . When Fraunhofer invented the spectroscope, he could observe and measure with his instrument dark (absorption) lines in the spectrum of sunlight. Thereafter, Kirchhoff and Bunsen discovered that elements cause bright (emission) lines when heated in a gas flame. The spectral lines are reliable indicators of chemical elements and became instrumental for the introduction of quantum theory [2] . Today’s spectrometers are sophisticated instruments that analyze electromagnetic radiation far beyond the narrow band of visible light and are extensively used in physics, chemistry, astrophysics, and astronomy [3] . Spectrometric analysis of electromagnetic radiation from remote stars, galaxies, and gas clouds allows determination of radiation intensity, as well as chemical composition of the objects and their velocity based on redshift [4] .
The Huygens-Fresnel principle illustrates how different speeds of wave propagation in different media cause refraction. A prism causes different degrees of refraction for different wave lengths and thus the dispersion (blue waves are more slowed down than red waves).
Bohr was first to try to explain the existence of spectral lines with his atomic model. Later corrections led to the quantum mechanical atomic model that fully explains the absorption and emission of specific wavelengths by specific elements (see Cloud of electrons, Note 1, for more details on the atomic model).
Modern spectrometers exploit both, wave and particle nature of electromagnetic radiation. High-energetic radiation (e.g. X-ray) is detected by Compton scattering, which evaluates particle properties. Low-energetic radiation (e.g. radio) is detected by electronic oscillator circuits, which evaluate wave properties.
Large telescopes are equipped with spectrographs that record detected spectral data (from radio to gamma rays, depending on the type of telescope). For remote galaxies and quasars a redshift of spectral lines is recorded, i.e. the lines are observed at longer wavelength than known from the Sun. The redshift is proportional to the (radial) velocity at which the object is moving away from us. Two interpretations are possible: Doppler effect, analog to the change of sound (longer waves) once an approaching car has passed the observer; or expansion of space that carries the galaxies with it, analog to a raising dough with raisins in it. Observations seem to indicate that for distances up to about 300 million lightyears the Doppler effect dominates and that for larger distances the expansion of space dominates.