Atomic and molecular materials can also absorb electromagnetic radiation. The set of wavelengths or frequencies of electromagnetic radiation absorbed by any single, pure material is unique to that material, and can be used as a "fingerprint" to identify the material. The record of the absorbed wavelengths or frequencies is an absorption spectrum.
The instrument used to measure the absorption spectrum of a material is called a spectrometer. Newton's experiment, illustrated in Figure 1, has all but one of the components of a simple absorption spectrometer: a sample placed between the light source and the prism. With a sample in place, some of the wavelengths of sunlight (consisting of all visible wavelengths) will be absorbed by the sample. Light not absorbed by the sample will, as before, be separated (dispersed) into its component wavelengths (colors) by the prism. The appearance of the spectrum will resemble that obtained without the sample in place, with the exception that those wavelengths which have been absorbed are missing, and will appear as dark lines within the spectrum of colors. If a piece of the photographic film is used instead of the card, the absorption spectrum can be recorded.
The absorption spectrum of gaseous hydrogen atoms recorded on a photographic plate is presented here. Atomic spectra recorded on photographic plates were among the earliest to be studied, and the appearance of these spectra led to the use of the term "line spectrum" to describe atomic spectra (either emission or absorption). The term is still commonly used even if the spectra are not recorded photographically.
Molecules also absorb electromagnetic radiation, but in contrast to atoms, molecules will absorb broader regions, or bands, of the electromagnetic spectrum. Molecular spectra are therefore often referred to as band spectra.
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