What does absorbance measure?
A useful number for how much a beam changes as it passes through a sample.
Compare light entering and leaving
Absorbance compares the radiant power entering a sample with the power transmitted through it. This site uses the base-10, or decadic, definition. Absorbance is dimensionless: it is a logarithm of a ratio, not a concentration or a percentage.
Absorption names a physical process in which matter takes up radiation. Absorbance is a defined numerical quantity. In the IUPAC definition, effects from the cell walls are excluded.
One step means ten times less transmission
Imagine an idealised beam with 100 units of incident power. If 10 units leave the sample, A = 1. If only 1 unit leaves, A = 2. A = 2 does not mean twice as much light is absorbed as A = 1.
These are arithmetic examples, not measured samples. The beam illustrations on this site show transmission at a selected wavelength; they do not predict the colour a solution will appear to your eye.
A reading needs context
A concentration calculation also needs a suitable absorption coefficient and path length, or a calibration made from known standards. Wavelength, sample conditions, blank measurements and instrument behaviour matter. A correct equation cannot establish that these are suitable for an experiment.
Sources & further reading
- IUPAC Gold Book — Absorbance
- IUPAC Gold Book — Beer–Lambert law
- Agilent — The Basics of UV-Vis Spectroscopy
Original educational material. Content review records a source and consistency check; it is not specialist peer review or laboratory method validation.