page_title: Ultraviolet–visible absorption
section_title: Beer’s Law
hierarchical_section_title: Ultraviolet–visible absorption / Beer’s Law
caption_attribution_description: English: Temperature-controlled Beckman DU640 spectrophotometer
context_page_description: UV-visible absorption is a process where a molecule absorbs ultraviolet or visible light that excites electrons. This energy causes an electronic transition from a ground state to an excited state.
  This is used in a technique of absorption spectroscopy known as UV-visible spectroscopy.
context_section_description: The absorption of a molecule can be used to determine the concentration of the molecule in solution.  To find concentration, Beer’s law is used.  This is a mathematical relationship shown by the equation below.  
  The components of Beer’s Law can be determined by tests. They can also be referenced in literature.  If the absorption is measured by a UV-visible spectrophotometer (instrument to measure absorption and wavelength of molecules), pathlength (b) is related to the sample container used in the test.  The sample container (named a cuvette) is made of a material that does not absorb light in this region.  The thickness of this cuvette is a known value. This is the pathlength the light will travel through to reach the sample. 
  The molar absorptivity coefficient can be calculated by measuring the absorption of a sample of a known concentration and known pathlength.  The absorptivity coefficient units are dependent on the pathlength and concentration units.  Molar absorptivity units are L mol-1 cm-1 when the concentration units are molarity.  The absorptivity is a measure of how strongly a molecule absorbs light at a particular wavelength.  Once this constant is determined, unknown concentrations can be determined from this relationship.
caption: Beckman DU640 UV/Vis spectrophotometer.
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