When a beam of electromagnetic radiations passes through a medium there may occur variations in its intensity which in turn vary with changes in wave length.
Similarly, when a substance is excited thermally or electrically it emits radiations of characteristic wave lengths. The instrument which is used to study this phenomenon is known as Spectrophotometer. There are two ways in which interactions of electromagnetic radiations with matter are usually observed:
1. Study of Absorption Spectra:
When radiations from a source which produces continuous beam of electromagnetic radiations are directed through a sample, the intensity of emergent radiations at different wave-lengths provides information about the Absorption Spectra. The graph of intensity plotted against different wave-lengths is referred to as Absorption Spectrum of the substance concerned.
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2. Study of Emission Spectra:
The intensity of radiations coming from the sample itself which is excited thermally or electrically at different wave lengths provides information about the Emission Spectra of the sample. The graph of intensity plotted against different wave lengths of radiations emitted by the sample is referred to as the Emission Spectra of the substance concerned.