What are the principles of absorption spectroscopy?

Absorption spectroscopy is based on the principle that every material or substance has a specific absorption spectrum, which is unique to the substance. Absorption spectrum refers to the range of radiation that the material absorbs at various frequencies. The atomic and molecular composition of a material determines its absorption spectrum. 

The frequency of light radiation that a material absorbs depends on the difference in energy between two energy states of the molecules. Electrons of an object absorb energy when a photon with sufficient energy reaches the object. The absorption of energy causes the electrons to move into a higher energy state. 

The amount of photon (radiation) absorbed results in an absorption spectrum which can then be measured in terms of absorbance. The absorbance of the sample can be used to determine the concentration of molecules in the sample.