What is Beer's law as used in spectrophotometry?

• A. A = e × b × c
• B. %T = I / I0 × 100
• C. E = hν
• D. e = ΔpH × 0.59V

Answer: (A)

Beer's law in spectrophotometry says that how much light a solution absorbs at a given wavelength is directly proportional to how much absorbing substance is present and how far the light travels through the solution. The equation expresses this as A = εbc, where A is absorbance, ε is the molar absorptivity (a constant for that substance and wavelength), b is the path length of the light through the sample (in cm), and c is the concentration (in mol/L). This linear relationship lets you determine concentration from a measured absorbance if you know ε and b (and stay within the linear range).

The other options don’t reflect Beer's law: percent transmittance is a related but different quantity (not Beer's law itself), E = hν is Planck’s relation for photon energy, and the last choice isn’t a valid equation used in this context.