Action Spectrum

The action spectrum measures something functionally different from the absorption spectrum: instead of asking how much light a pigment absorbs, it asks how effective each wavelength of light actually is at driving photosynthesis, measured as photosynthetic rate (or quantum yield) plotted against wavelength. From this curve, it is clear that photosynthesis proceeds fastest when illuminated with blue or red light, closely mirroring where chlorophyll's own absorption peaks lie - direct evidence that chlorophyll absorption really is what powers photosynthesis, rather than some other coincidental pigment. T. W. Engelmann was the first to plot an action spectrum of photosynthesis, in 1888. Comparing the action spectrum against the absorption spectrum in detail turned out to be more than just a confirmation exercise - subtle discrepancies between the two curves, especially toward the far-red end of the spectrum, were what first hinted that a single photosystem could not fu …