What does absorbance mean?
Absorbance is a scientific measure of how much light a substance absorbs as light passes through it. Defined formally as the negative logarithm of transmittance, it expresses, on a convenient logarithmic scale, the degree to which a sample attenuates a beam of radiation. The term belongs to physics, chemistry, and biology laboratories rather than everyday speech: spectrophotometers report absorbance directly, and because absorbance is proportional to the concentration of the absorbing species (the relationship known as Beer's law), it underpins countless routine measurements, from assaying DNA purity at 260 nanometres to monitoring enzyme reactions. It is a dimensionless quantity, typically written simply as A and often qualified by wavelength. Closely related terms include optical density, an older near-synonym still common in some fields, and transmittance, its logarithmic counterpart. For anyone reading experimental results, absorbance is one of those quiet workhorse terms that makes invisible chemistry visible in a single number.
nounA measure of the amount of light absorbed by a substance as light passes through it, defined mathematically as the negative logarithm of transmittance. It is the quantity read directly by a spectrophotometer and is central to quantitative chemical analysis.
- In physics and chemistry: the logarithmic measure of the fraction of light (or other radiation) absorbed by a sample as it passes through it, equal to the negative base-ten logarithm of transmittance.
"The protein concentration was calculated from the absorbance measured at 280 nanometres."
"The technician recorded an absorbance of 0.45 at a wavelength of 540 nanometres."
"Because absorbance is proportional to concentration, diluting the sample twofold halved the reading on the spectrophotometer."
Rarely pluralised; the plural appears mainly when comparing readings across samples or wavelengths.
"The absorbances of the standards were plotted against their concentrations to build the calibration curve."
Double the concentration, double the absorbance — that simple logarithmic relationship, Beer's law, is why scientists can weigh invisible molecules just by shining light through them.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of absorbance
Absorbance derives from the verb absorb, which English took from Latin absorbere, formed from ab- ('away, from') and sorbere ('to suck in, swallow'). The noun was coined in the nineteenth century alongside related optical terms as scientists quantified how materials take up light. It shares this Latin root with absorption, absorbent, and ultimately sorption-related vocabulary, all tracing back to the image of drinking in or swallowing.
How absorbance is actually used
Absorbance is a technical term confined almost entirely to laboratory and scientific writing — spectroscopy, analytical chemistry, molecular biology. It is a dimensionless quantity, conventionally reported without units, though older literature may label it 'optical density' or 'OD'. In formal scientific prose it is usually abbreviated A and qualified by wavelength, as in 'A260' for nucleic acid measurement.
Easily confused with absorbance
Absorbency is the capacity of a material such as paper or fabric to soak up liquid, whereas absorbance is a physical measure of how much light a sample absorbs.
Absorption is the general process by which matter takes in light or another substance, while absorbance is the specific numerical quantity used to quantify that uptake of light.