Beer-Lambert Law — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementBeer-Lambert LawFRCA Primary

A pulse oximeter estimates arterial oxygen saturation by measuring the attenuation of red and infrared light transmitted through tissue. The Beer–Lambert law, which underpins this measurement, describes:

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Correct answer: B — The relationship between light absorption and concentration of a substance in solution

Explanation lettering: E = shown as A · C = shown as B · A = shown as C · B = shown as D · D = shown as E

C is correct. The Beer–Lambert law combines Beer's law (absorbance is proportional to the concentration of the absorbing substance) with Lambert's law (absorbance is proportional to the path length), giving A = εcl. Because oxyhaemoglobin and deoxyhaemoglobin absorb red (660 nm) and infrared (940 nm) light differently, comparing pulsatile absorbance at the two wavelengths allows the pulse oximeter to estimate SpO2; the same principle underlies co-oximetry and capnography by infrared absorption. The distractors describe other named laws relevant to anaesthetic physics: A is Boyle's law (pressure–volume at constant temperature), B is Charles's law (volume–temperature at constant pressure), D is Henry's law (amount of gas dissolved is proportional to its partial pressure, relevant to blood-gas solubility), and E is the Hagen–Poiseuille relationship governing laminar flow through a tube. None of these concerns light absorption.

Reference: Oshina I, Spigulis J. Beer-Lambert law for optical tissue diagnostics: current state of the art and the main limitations. Journal of Biomedical Optics, 2021. https://pubmed.ncbi.nlm.nih.gov/34713647/