Spectrophotometry is the quantitative method of evaluating the radiation interacting with a material by measuring its relative intensity.

This includes radiation transmitted through, reflected from, diffracted within, absorbed by, or emitted by the material.

In an absorption spectrometer, electromagnetic radiation of a particular wavelength is passed through the sample to a detector. Radiation absorbed by the sample yields a reduced radiation intensity at the detector. This process is then repeated for a range of wavelengths.

An absorption spectrum is the plot of the intensity lost to absorbed radiation versus the radiation wavelength.

Similarly, plotting the diffracted or transmitted radiation intensities against the wavelength generates the diffraction and transmission spectra.

In a molecule, the energy gaps for different types of transitions fall into distinct ranges.

So, the range of wavelengths indicates the transition type that occurred, and the specific wavelength of absorption can give an idea about which functional group may be in the molecule.