5.6—Absorption and action spectra

An absorption spectrum shows which wavelengths a pigment absorbs. An action spectrum shows how effective different wavelengths are at driving photosynthesis, usually inferred from oxygen production or another rate measure.

The two spectra answer different questions: absorption describes light capture, whereas action describes the biological response. A peak in action can reflect combined pigments and electron-transfer efficiency rather than one pigment alone.

If photosynthetic rate peaks under blue and red light but falls under green light, the action spectrum indicates effectiveness; a pigment absorption spectrum helps explain why those wavelengths are captured differently.

An absorption peak is not automatically a photosynthesis-rate peak. Keep wavelength, pigment absorbance and measured rate as separate variables.