The ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a spectrophotometer. This machine directs beams of light of different wavelengths through a solution of the pigment and measures the fraction of the light transmitted at each wavelength. A graph plotting a pigment’s light absorption versus wavelength is called an absorption spectrum.
The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis, since light can perform work in chloroplasts only if it is absorbed. the absorption spectra of three types of pigments in chloroplasts:
chlorophyll a, the key light-capturing pigment that participates directly in the light reactions;
the accessory pigment chlorophyll b; and a separate group of accessory
pigments called carotenoids. The spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis, since they are absorbed, while green is the least effective color. This is confirmed by an action spectrum for photosynthesis, which profiles the relative
effectiveness of different wavelengths of radiation in driving
the process.
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