Beer-Lambert law
/ BEER LAM-bert law /
Imagine sunlight passing through stacked sheets of tinted glass. Each sheet removes the same fraction of the light, so the more sheets — or the darker the tint — the dimmer the beam that comes out. Beer-Lambert law is the tidy rule that captures this: light fades in a predictable way as it passes through an absorbing material.
Stated as a formula, A = ε c l: the absorbance (A) equals the molar absorptivity (ε, how strongly the substance grabs that colour of light) times the concentration (c) times the path length (l, how far the light travels through the sample). Because A rises in step with concentration, a single absorbance reading can be turned straight into a concentration.
This straight-line relationship is what makes spectrophotometry so useful, but it is only an approximation. At high concentrations molecules interfere with one another, stray light leaks in, and the line bends, so the law works best for dilute, clear solutions read at a well-chosen wavelength.
A dye solution in a 1 cm cell gives an absorbance of 0.40; dilute it to half strength and the absorbance drops to about 0.20, exactly as A = ε c l predicts when c is halved.
Halving the concentration halves the absorbance — the heart of the law.
Deviations from the straight line at high absorbance are normal, not instrument faults. Keep absorbance roughly between 0.1 and 1.0 for the most reliable results.