A spectroscope is an instrument used to observe the atomic spectrumof a given material. Because atoms can absorb or emit radiation only at certain specific wavelengths defined by electron transitions, the spectrum of each type of atom is directly related to its structure. There are two classifications of atomic spectra: absorption and emission.

An absorption spectrum is produced when light passes through a cool gas. From quantum mechanics we know that the energy of light is directly proportional to its wavelength. For a given type of atom, a photon of light at some specific wavelength can transfer its energy to an electron, moving that electron into a higher energy level. The atom is then in an "excited state." The electron absorbs the energy of the photon during this process. Thus, a white light spectrum will show a dark line where light of that energy/wavelength has been absorbed...