There are three types of spectra emitted by objects:
Continuous spectrum - a solid or liquid body radiates an uninterrupted, smooth spectrum (Planck curve)
Emission spectrum - a radiating gas produces a spectrum of discrete spectral lines
Absorption spectrum - a continuous spectrum that passes through a cool gas has specific spectral lines removed (inverse of an emission spectrum)
Just to complement Kirchhoff's three laws of spectroscopy:
In 1835 Charles Wheatstone showed that the light from metals in an electric arc produced characteristic rays when passed through a prism.
In 1854, credit for the origins of Spectrum Analysis goes to Dr. David Alter, a scientist of Freeport, Pennsylvania, who published the first scientific work that included the spectral radiance properties for twelve metals, titled:
On Certain Physical Properties of Light Produced by the Combustion of Different Metals in an Electric Spark Refracted by a Prism.
Dr. Alter began studying the optical properties of matter ever since finding a piece of melted, prismatic glass in the debris of the great Pittsburgh fire of 1845. By 1855, Alter published another article that expanded his original theory by including six gases, including the first discovery of what came to be named the Balmer lines of hydrogen. The proof that elemental gases have spectra peculiar to themselves was an extremely important scientific advance. Alter's article contains a paragraph where he even visualized the application of spectrum analysis to astronomy, mentioning the study and detection of elements in the combustion of shooting-stars or luminous meteors, and daguerreotyped the dark lines of the solar spectrum David's spectral discoveries were noted in various scientific publications in France, Germany and Switzerland from 1854 to 1860.
Anders Jonas Ångström a physicist in Sweden, in 1853 had presented similar theories about gases having spectra in his work: Optiska Undersökningar to the Royal Swedish Academy of Sciences pointing out that the electric spark yields two superposed spectra. Ångström also postulated that an incandescent gas emits luminous rays of the same refrangibility as those which it can absorb. This statement contains a fundamental principle of spectrum analysis.
In 1860, German physicist Gustav Kirchhoff and chemist Robert Bunsen published their own findings on the spectra of eight metals and identified these metals in natural elements. Kirchoff went on to contribute fundamental research on the nature of spectral absorption. Spectrum analysis was then grouped by Kirchhoff into the three fundamental laws commonly called Kirchoff's Laws, these laws integrated both Alter and Ångström's discoveries of radiance and emission with Kirchhoff's fundamental discoveries of absorption.
Johann Balmer discovered in 1885 that the four visible lines of hydrogen were part of a series which could be expressed in terms of integers. This was followed a few years later by the Rydberg formula describing additional series of lines.
In the early twentieth century, spectrum analysis led to "atomic spectroscopy" and quantum mechanics.
