Spectroscopy is the study of the interaction between matter and electromagnetic radiation. Spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism; the concept was expanded to include any interaction with radiative energy as a function of its wavelength or frequency, predominantly in the electromagnetic spectrum, though matter waves and acoustic waves can be considered forms of radiative energy. Spectroscopic data are represented by an emission spectrum, a plot of the response of interest, as a function of wavelength or frequency. Spectroscopy in the electromagnetic spectrum, is a fundamental exploratory tool in the fields of physics and astronomy, allowing the composition, physical structure and electronic structure of matter to be investigated at atomic scale, molecular scale, macro scale, over astronomical distances. Important applications arise from biomedical spectroscopy in the areas of tissue analysis and medical imaging. Spectroscopy and spectrography are terms used to refer to the measurement of radiation intensity as a function of wavelength and are used to describe experimental spectroscopic methods.

Spectral measurement devices are referred to as spectrometers, spectrophotometers, spectrographs or spectral analyzers. Daily observations of color can be related to spectroscopy. Neon lighting is a direct application of atomic spectroscopy. Neon and other noble gases have characteristic emission frequencies. Neon lamps use collision of electrons with the gas to excite these emissions. Inks and paints include chemical compounds selected for their spectral characteristics in order to generate specific colors and hues. A encountered molecular spectrum is that of nitrogen dioxide. Gaseous nitrogen dioxide has a characteristic red absorption feature, this gives air polluted with nitrogen dioxide a reddish-brown color. Rayleigh scattering is a spectroscopic scattering phenomenon. Spectroscopic studies were central to the development of quantum mechanics and included Max Planck's explanation of blackbody radiation, Albert Einstein's explanation of the photoelectric effect and Niels Bohr's explanation of atomic structure and spectra.

Spectroscopy is used in physical and analytical chemistry because atoms and molecules have unique spectra. As a result, these spectra can be used to detect and quantify information about the atoms and molecules. Spectroscopy is used in astronomy and remote sensing on Earth. Most research telescopes have spectrographs; the measured spectra are used to determine the chemical composition and physical properties of astronomical objects. One of the central concepts in spectroscopy is its corresponding resonant frequency. Resonances were first characterized in mechanical systems such as pendulums. Mechanical systems that vibrate or oscillate will experience large amplitude oscillations when they are driven at their resonant frequency. A plot of amplitude vs. excitation frequency will have a peak centered at the resonance frequency. This plot is one type of spectrum, with the peak referred to as a spectral line, most spectral lines have a similar appearance. In quantum mechanical systems, the analogous resonance is a coupling of two quantum mechanical stationary states of one system, such as an atom, via an oscillatory source of energy such as a photon.

The coupling of the two states is strongest when the energy of the source matches the energy difference between the two states. The energy of a photon is related to its frequency by E = h ν where h is Planck's constant, so a spectrum of the system response vs. photon frequency will peak at the resonant frequency or energy. Particles such as electrons and neutrons have a comparable relationship, the de Broglie relations, between their kinetic energy and their wavelength and frequency and therefore can excite resonant interactions. Spectra of atoms and molecules consist of a series of spectral lines, each one representing a resonance between two different quantum states; the explanation of these series, the spectral patterns associated with them, were one of the experimental enigmas that drove the development and acceptance of quantum mechanics. The hydrogen spectral series in particular was first explained by the Rutherford-Bohr quantum model of the hydrogen atom. In some cases spectral lines are well separated and distinguishable, but spectral lines can overlap and appear to be a single transition if the density of energy states is high enough.

Named series of lines include the principal, sharp and fundamental series. Spectroscopy is a sufficiently broad field that many sub-disciplines exist, each with numerous implementations of specific spectroscopic techniques; the various implementations and techniques can be classified in several ways. The types of spectroscopy are distinguished by the type of radiative energy involved in the interaction. In many applications, the spectrum is determined by measuring changes in the intensity or frequency of this energy; the types of radiative energy studied include: Electromagnetic radiation was the first source of energy used for spectroscopic studies. Techniques that employ electrom

The Hot Metal Bridge is a truss bridge in Pittsburgh, that crosses the Monongahela River. The bridge consists of two parallel spans on a single set of piers: the former Monongahela Connecting Railroad Bridge, built in 1887, on the upstream side and the former Hot Metal Bridge, built in 1900, on the downstream side; the Monongahela Connecting Railroad Bridge carried conventional railroad traffic, while the Hot Metal Bridge connected parts of the J&L Steel mill, carrying crucibles of molten iron from the blast furnaces in ladle transfer cars to the open hearth furnaces on the opposite bank to be converted to steel. During World War II 15% of America's steel making capacity crossed over the Hot Metal Bridge, up to 180 tons per hour; the upstream span was converted to road use after a $14.6 million restoration, opened by Mayor Murphy with a ceremony honoring former steel workers on June 23, 2000. The bridge connects 2nd Avenue at the Pittsburgh Technology Center in South Oakland with Hot Metal Street in the South Side.

The downstream span reopened for pedestrian and bicycle use in late 2007 after two years of work. The Great Allegheny Passage hiker/biker trail passes over this bridge as it approaches Pittsburgh's Golden Triangle area; the Pittsburgh History and Landmarks Foundation was responsible for managing the decorative lighting project for the bridge, lit with energy-efficient light-emitting diode and optical fiber technology on June 12, 2008. The Hot Metal Bridge was inducted into the North America Railway Hall of Fame in 2016; the Hot Metal Bridge is the namesake of the defunct Hot Metal Grille at the nearby SouthSide Works shopping center and is the name of the online magazine of the University of Pittsburgh, a literary magazine. The bridge was in several scenes of the 2011 film Warrior starring Nick Nolte. List of bridges documented by the Historic American Engineering Record in Pennsylvania List of crossings of the Monongahela River Pittsburgh bridges Great Allegheny Passage Pig iron Historic American Engineering Record No.

PA-277-B, "Monongahela Connecting Railroad Company, Main Bridge, Spanning Monongahela River at mile post 3.1, Allegheny County, PA", 12 photos, 2 data pages, 2 photo caption pages HAER No. PA-277-C, "Monongahela Connecting Railroad Company, Hot Metal Bridge, Spanning Monongahela River at mile post 3.1, Allegheny County, PA", 13 photos, 4 data pages, 2 photo caption pages Hot Metal Bridge at pghbridges.com Pittsburgh Post-Gazette - Official reopening of the bridge after the conversion Pittsburgh Post-Gazette - Article on conversion of bridge to pedestrian and bicycle use Pittsburgh Post-Gazette - High bids threaten the conversion project Pittsburgh Post-Gazette - Groundbreaking on the new project

The Aschaffenburg−Höchst railway was a single-tracked, Bavarian branch line in southern Germany, running from Aschaffenburg to the Odenwald railway at Höchst im Odenwald. Because the railway ran through the Bachgau, it was known as the Bachgaubahn; the 30 kilometre long railway was built under the terms of the Lokalbahn law of 12 July 1906 and a state treaty between the Kingdom of Bavaria and the Grand Duchy of Hesse. On 1 May 1911 the first section between Aschaffenburg-Süd und Großostheim was taken into service. In 1973 the Minister for Transport authorised the closure of the line as part of the package of savings measures proposed by the Deutsche Bundesbahn. On 25 May 1974 passenger trains ran for the last time and all traffic between Großostheim and Neustadt was ceased. In the following weeks and months the tracks were lifted on the stretch between Großostheim and Neustadt and the 5 kilometre long section between Neustadt and Höchst placed under the Frankfurt railway division Services ceased here at the end of 1992 and the line was dismantled in 1999.

Goods traffic between Aschaffenburg-Nilkheim and Großostheim came to an end on 28 September 1991 and the remaining section of line with its siding to the Port of Aschaffenburg was converted operationally into a station track. In the Großostheim districts of Wenigumstadt and Pflaumheim the former trackbed has been a protected site since 1997; the most striking structure on the route, the viaduct over the village of Höchst, still exists and continues to dominate the local scene. The former locomotive shed at Höchst station is nowadays a private residence. Further on, sections of the old track are still visible as far as Neustadt. Between Neustadt and Hainstadt, the former railway embankment and a bridge can be made out; the station building in Sandbach is a private property. On the old station yard at Sandbach the firm of Pirelli has built a workshop and on the former trackbed there is now a link road between the two tyre factories. Houses and a supermarket have been built on the old station yard in Neustadt.

At the beginning of spring 2008 the tracks between the Höchster viaduct and Sandbach were removed and the old level crossing filled in with concrete. Local public transport services are being expanded by private initiatives in view of the future economic development of the Odenwald area. Wolfram Alster, the former head of the now-disbanded Überwaldbahn has indicated that a reactivation of the Bachgaubahn would be possible. Andreas Kuhfahl: Nebenbahnen in Unterfranken. Eisenbahn-Fachbuchverlag Neustadt/Coburg, 2003. ISBN 3-9805967-9-6 Alois Ott, Hans H. Weber: Verkehrseinrichtungen der Gemeinde Wenigumstadt. In: Wenigumstadt – Beiträge zur Geschichte einer Bachgaugemeinde. Wenigumstadt 1977. Frank Schmelz: Lineare anthropogene Gehölz- und Saumstrukturen im Bachgau, Gießen 2001