Let’s start with the raw observation. If you shine light of different colours on a leaf and measure how much of that light gets absorbed, you get one curve. If you instead measure how much photosynthesis happens under each colour, you get a second curve. The two curves look very similar — both show high activity in blue and red light, and a dip in green. But they are not the same thing, and the difference between them is the whole point.
The intuition: what each spectrum actually measures
Think of a solar panel. The absorption spectrum tells you which wavelengths of sunlight the panel can capture — which colours it “soaks up.” The action spectrum tells you how much electrical power the panel actually produces at each wavelength. A panel might absorb a lot of green light but convert it poorly into electricity; the action spectrum would then be low in green even though absorption is high.
In a leaf, the pigments (chlorophyll a, chlorophyll b, carotenoids) are the “solar panels.” The absorption spectrum is a property of the pigments themselves — it shows what fraction of each wavelength they can physically take in. The action spectrum is a property of the whole photosynthetic machinery — it shows the rate of oxygen evolution (or CO₂ fixation) at each wavelength.
The precise statements
Absorption spectrum: A graph of the fraction of incident light absorbed (or the absorbance) by a pigment or a leaf, plotted against wavelength.
Action spectrum: A graph of the rate of photosynthesis (e.g., O₂ released per minute) plotted against wavelength.
For a leaf containing the usual mix of pigments, the absorption spectrum peaks in the blue (around 430–450 nm) and red (around 640–680 nm) regions, with a trough in green (around 500–600 nm). That is why leaves look green — they reflect and transmit green light instead of absorbing it.
The action spectrum for photosynthesis peaks in the same blue and red regions. This is no coincidence: the light that gets absorbed is the light that drives the light reactions. But the match is not perfect. In the blue region, a significant fraction of the absorbed energy is dissipated as heat or fluorescence rather than used for photochemistry, so the action spectrum in blue is slightly lower relative to the absorption spectrum than you might expect. In the red region, the efficiency is higher.
Why the two spectra are taught together
The comparison proves a fundamental point: only the light that is absorbed can drive photosynthesis. If a pigment does not absorb a certain wavelength, that wavelength cannot contribute to the light reactions. That is why green light is so poorly used — not because it is “bad” light, but because the main pigments simply do not capture it well.
A common exam question asks: “Why is the action spectrum not identical to the absorption spectrum?” The answer is that not all absorbed photons are equally effective — some energy is lost as heat or fluorescence, and accessory pigments (like carotenoids) transfer energy to chlorophyll with less than 100% efficiency.
The graph you will see …