Absorbance formula as given?

• A. A = -log T
• B. A = log %T
• C. A = 2 - log%T
• D. A = log(1/ %T)

Answer: (C)

Absorbance is defined as the negative base-10 logarithm of transmittance. When transmittance is a decimal value (0 to 1), this is A = -log10(T). If transmittance is given as a percent, %T, then T = %T/100. Substituting yields A = -log10(%T/100) = -log10(%T) + log10(100) = 2 - log10(%T). So the expression A = 2 − log%T is the correct form when using percent transmittance, since the 2 accounts for the conversion from percent to a decimal fraction.