Spectroscope in the context of Reciprocal centimeters

An Amici prism, named for the astronomer Giovanni Battista Amici, is a type of compound dispersive prism used in spectrometers. The Amici prism consists of two triangular prisms in contact, with the first typically being made from a medium-dispersion crown glass, and the second from a higher-dispersion flint glass. Light entering the first prism is refracted at the first air–glass interface, refracted again at the interface between the two prisms, and then exits the second prism at near-normal incidence. The prism angles and materials are chosen such that one wavelength (colour) of light, the centre wavelength, exits the prism parallel to (but offset from) the entrance beam. The prism assembly is thus a direct-vision prism and is commonly used as such in hand-held spectroscopes. Other wavelengths are deflected at angles depending on the glass dispersion of the materials. Looking at a light source through the prism thus shows the optical spectrum of the source.

By 1860, Amici realized that one can join this type of prism back-to-back with a reflected copy of itself, producing a three-prism arrangement known as a double Amici prism. This doubling of the original prism increases the angular dispersion of the assembly and also has the useful property that the centre wavelength is refracted back into the direct line of the entrance beam. The exiting ray of the center wavelength is thus not only undeviated from the incident ray, but also experiences no translation (i.e. transverse displacement or offset) away from the incident ray's path.

Indium is a chemical element; it has symbol In and atomic number 49. It is a silvery-white post-transition metal and one of the softest elements. Chemically, indium is similar to gallium and thallium, and its properties are largely intermediate between the two. It was discovered in 1863 by Ferdinand Reich and Hieronymous Theodor Richter by spectroscopic methods and named for the indigo blue line in its spectrum.

Indium is used primarily in the production of flat-panel displays as indium tin oxide (ITO), a transparent and conductive coating applied to glass. It is also used in the semiconductor industry, in low-melting-point metal alloys such as solders and soft-metal high-vacuum seals. It is used in the manufacture of blue and white LED circuits, mainly to produce Indium gallium nitride p-type semiconductor substrates. It is produced exclusively as a by-product during the processing of the ores of other metals, chiefly from sphalerite and other zinc sulfide ores.

Almandine (/ˈælməndɪn/), also known as almandite, is a mineral belonging to the garnet group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron aluminium garnet, of deep red color, inclining to purple. It is frequently cut with a convex face, or en cabochon, and is then known as carbuncle. Viewed through the spectroscope in a strong light, it generally shows three characteristic absorption bands.

Almandine is one end-member of a mineral solid solution series, with the other end member being the garnet pyrope. The almandine crystal formula is: Fe₃Al₂(SiO₄)₃. Magnesium substitutes for the iron with increasingly pyrope-rich composition.

Joseph Ritter von Fraunhofer (/ˈfraʊnˌhoʊfər/; German: [ˈfraʊnˌhoːfɐ]; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He developed diffraction grating and also invented the spectroscope. In 1814, he discovered and studied the dark absorption lines in the spectrum of the sun now known as Fraunhofer lines.

The German research organization Fraunhofer Society, which is Europe's biggest Society for the advancement of applied research, is named after him. Fraunhofer lines are used in astronomy to determine the composition of celestial bodies. His epitaph reads Aproximavit sidera, Latin for 'He brought closer the stars.'