Which fundamental law is used in spectrophotometry?

• A. Pascal's Law
• B. Beer’s Law
• C. Boyle's Law
• D. Newton's Law

Answer: (B)

Spectrophotometry relies on Beer’s Law, which states that the amount of light absorbed by a solution is proportional to both the concentration of the absorbing species and the path length the light travels through the sample. In practice, you measure how much light at a specific wavelength is transmitted, and absorbance is defined as A = -log10(I/I0). The proportionality constant ε (molar absorptivity) is specific to the substance and wavelength, so A = εlc lets you determine concentration from the measured absorbance when l is known, often using a calibration curve. This linear relationship is what makes spectrophotometry a quantitative method. The other laws describe pressure-volume behavior, fluid pressure, or motion, which don’t explain how light is absorbed by a solution, so they aren’t used to relate light measurements to concentration.