How is spectroscopy used?

All spectroscopy methods use light wavelengths from infrared to UV. The wavelength used will correspond to different characteristics of molecules and atoms.
Adsorption spectroscopy measures the amount of a wavelength adsorbed by a substance. Emission spectroscopy measures the amount of a wavelength emitted by a substance after excitation. In both versions a molecule is interacted with by external light or energy and the way that energy is used or transformed by the molecule is specific to its structure.
IR-frequency light is passed through a compound. The amount and frequencies of the light absorbed is related to the functional groups and structure of the compound. This helps us to identify the compound.
in organic chemistry is most often used to identify chemical structures from the characteristic adsorption of IR light by different organic functional groups. Thus the existence of hydroxyl groups double bonds and other active parts of a molecule can be deduced.