The Language of Spectroscopy
When a spectrophotometer shoots a beam of light through a chemical sample, the raw data it collects is Transmittance—the physical percentage of light photons that successfully survived the journey through the liquid and hit the detector on the other side.
However, Transmittance is an exponential curve. If you double the concentration of your liquid, the transmittance does not neatly drop by half.
Why We Use Absorbance
Because humans and basic statistics prefer straight lines, chemists mathematically transform the exponential Transmittance data into a logarithmic scale called Absorbance.
Absorbance is perfectly linear. If you double the concentration of the liquid, the Absorbance exactly doubles. This linear relationship is what makes the Beer-Lambert Law work.
The Conversion Formula
To perform this conversion, you must first convert the Percentage Transmittance (e.g., 50%) into a fractional decimal (e.g., 0.50), and then take the negative base-10 logarithm.