In Beer's Law, D is distance; C concentration; a constant; e natural log. Which option represents distance in the exponent?

• A. C
• B. a
• C. e
• D. D

Answer: (D)

Beer's Law shows that light decreases exponentially as it travels through a sample. When written in the exponential form, the transmitted intensity is proportional to e to the negative of absorptivity times concentration times the distance the light travels (path length). Because the relationship is exponential, the distance factor must appear inside that exponent. That distance component is the path length, which in this question is the distance variable. The other symbols correspond to concentration (how much solute is present) or constants (such as absorptivity) or the base of the natural logarithm, not distance. So the distance in the exponent is the distance the light travels, denoted here by D.