UV–visible spectroscopy
/ U-V VIZ /
The reason a leaf is green or a sky-blue dye is blue is that the substance has absorbed some colours of visible light and let the rest through. UV–visible spectroscopy is the instrument-grade version of noticing colour: it shines ultraviolet and visible light across a sample, one wavelength at a time, and records which ones get absorbed — turning the bare fact 'it looks blue' into a precise curve.
Technically, this technique measures the absorption of ultraviolet and visible light, which carries enough energy to lift electrons from lower-energy orbitals into higher ones. The position of an absorption peak tells you about the molecule's electronic structure — particularly its system of double bonds and any light-grabbing groups — while the height of the peak, through the Beer–Lambert law, tells you how much of the substance is present.
It earns its place as one of the most common lab tools because it is quick, cheap, and excellent for measuring concentration of coloured or UV-active compounds. The caveat is that its peaks are broad and not very specific: many different molecules absorb in overlapping regions, so UV–visible is far better at saying 'how much' of a known substance than at identifying an unknown one on its own.
Chlorophyll extracted from spinach shows two strong absorption peaks — one in the blue, one in the red — while reflecting the green light in between, which is exactly why leaves look green. Measuring the height of one peak lets a biologist work out how concentrated the chlorophyll extract is.
Which colours get absorbed reveals structure; how much, reveals concentration.
UV–visible probes electrons jumping between orbitals, the largest energy gaps in a molecule, while infrared probes much smaller vibrational gaps. The two see completely different aspects of the same molecule and are routinely used together.