In Beer's law, absorbance A = ε b c. What are the typical units of path length b?

• A. meters
• B. dimensionless
• C. liters
• D. centimeters

Answer: (D)

Beer's law links how much light is absorbed to how far the light travels through the sample, so the path length must be in units that cancel with the units of the other factors. When using the common form where ε is given in L/(mol·cm) and the concentration c is in mol/L, the path length b needs to be in centimeters. This makes ε × b × c dimensionless, giving the absorbance A.

In practice, this is why standard cuvettes have a 1 cm path length—the numbers line up neatly and give a direct, easy calculation. If you used meters, you’d have to adjust ε to units of L/(mol·m) or convert b to cm.

So the typical units for the path length are centimeters.