In Beer's law, if path length doubles and concentration stays the same, what happens to absorbance?

• A. Absorbance doubles.
• B. Absorbance halves.
• C. Absorbance remains the same.
• D. Absorbance quadruples.

Answer: (A)

Absorbance depends directly on how far light travels through the sample and how many absorbing molecules are present, described by A = ε l c. If the path length doubles while the concentration stays the same, the absorbance doubles as well: A' = ε (2l) c = 2A. The molar absorptivity ε remains constant for a given wavelength, so nothing offsets the change. This linear relationship holds within Beer's law’s valid range; deviations can occur at very high absorbances, but the direct proportionality to path length explains why absorbance doubles here.