spectrophotometry
/ spek-troh-foh-TOM-eh-tree /
Hold a glass of strong tea up to a window and a glass of weak tea beside it. The strong one swallows more light and looks darker. Spectrophotometry simply turns that everyday judgement into a number: it measures how much light a sample soaks up, and uses that to figure out how much coloured stuff is dissolved inside.
More precisely, spectrophotometry is the technique of shining light of a chosen colour (wavelength) through a sample and measuring how much passes through versus how much is absorbed. Because many substances absorb a fixed share of light per unit of concentration, the reading tells you the concentration of that substance.
It is one of the workhorses of the analytical lab: cheap, fast, and gentle on the sample. Its main limit is that it only sees substances that absorb light at the wavelength you pick, and the simple proportionality between absorbance and concentration holds only over a limited range, so very concentrated samples must usually be diluted first.
To measure the iron in tap water, a chemist adds a reagent that turns the iron orange, then reads the absorbance at 510 nm; comparing it with a calibration curve gives the iron concentration in milligrams per litre.
Colour intensity, measured as absorbance, is converted into concentration.
Spectrophotometry is the broad method; the box that performs it is the spectrophotometer, and Beer-Lambert law is the equation that links its reading to concentration.