Ultraviolet–visible absorption
Absorption of light starts with energy of a certain wavelength in this UV-visible region being exposed to a molecule.  The light/energy then excites the ground state (non excited) outer or valence electrons to an excited state (high energy).  The outcome of this can be measured by a UV-visible spectrophotometer.  The data is shown as a spectrum with absorption versus wavelength.  This pattern can be used to learn properties of the molecule.  Only certain molecules can absorb light in this region.