Ultraviolet-visible (UV-vis) spectroscopy
Our eyes are visible energy spectrometers of a sort. We see light in the region of about 400-800 nanometers wavelength (frequency of 750-375,000,000,000,000,000 Hertz = sec-1). When light of a particular wavelength is absorbed, the remaining light is reflected or passed through the sample, giving the colors we see. If the wavelengths become somewhat shorter, humans cannot see such ultraviolet (UV) light, although we see the results as skin-tanning and sunburns. Typical instruments in the UV-vis range (spectrometers) can measure the 200-1000 nanometer range of the spectrum (at <200 nanometers, oxygen in the air absorbs the light, making measurement more difficult).
A UV-vis spectrophotometer
UV-vis spectroscopy is very sensitive for some molecules, and can detect less than one-millionth of a mole of compound in a liter of solvent! It is very easy to carry out for any sample that will transmit light (opaque or highly reflective samples will not work). It is very easy and convenient for quantitative measurements, using spectrometers that presently cost in the range of US$6000-30,000. Usually samples are dissolved in UV-transparent solvents, thin films, or gases in tubes.
However, UV-vis spectroscopy only gives very basic information about molecular structure. UV-vis active molecules must have easily deformed electron clouds. This requirement is met for molecules with lots of pi-bonds that are directly connected together, or conjugated. The more conjugated a system, the more its UV-vis spectrum is shifted to longer wavelengths (red-shifted). Examples of highly conjugated systems are chlorophyll-a (deep green color) and retinal (reddish color). Sometimes a combination of absorption peaks will give very dark colors.
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Unfortunately, UV-vis spectra for solutions are usually quite broad (10-100 nanometer peak width), so that molecules with very different structures may have similar UV-vis spectra. An example spectrum for chlorophyll extracted from plants by high school students of the UMass Science Enrichment Program is shown. Note that energy decreases from left to right.
Overall, UV-vis spectra give information about whether a molecule is highly conjugated or not. Other information may be guessed, but not usually determined with certainty.