In Beer-Lambert law, absorbance is proportional to what?

• A. Absorbance inversely proportional to concentration
• B. Absorbance proportional to wavelength
• C. Absorbance proportional to concentration and path length
• D. Absorbance independent of path length

Answer: (C)

The main idea being tested is how absorbance scales with the amount of absorbing material and the distance the light travels through it. In Beer-Lambert law, absorbance A equals ε times l times c (A = εlc), where ε is the molar absorptivity at a given wavelength, l is the path length the light travels through the sample, and c is the concentration.

Because ε is set for a specific wavelength and substance, absorbance is directly proportional to both concentration and path length. If you increase the concentration of the absorbing species or lengthen the path the light travels, more photons are absorbed, so A goes up. This linear relationship holds within the Beer-Lambert linear range.

So the best description is that absorbance is proportional to concentration and path length (with the proportionality constant depending on the wavelength). It is not inversely related to concentration, not simply proportional to wavelength (since ε varies with wavelength and the full relationship includes both c and l), and it certainly is not independent of path length.