Ultraviolet–visible absorption
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.