Photosynthetic pigments absorb specific wavelengths, with chlorophyll absorbing red and blue light strongly while reflecting or transmitting much green light.
Coverage 2014–2020 · Updated 15 Jul 2026
An absorption spectrum shows the proportion of each wavelength absorbed by a pigment; an action spectrum shows the measured rate or effectiveness of photosynthesis at each wavelength.
Only photons with suitable energies are absorbed and excite electrons in pigment molecules. Chlorophylls and accessory pigments absorb different wavelength ranges, so their combined absorption helps explain the action spectrum.
Plot wavelength in nanometres, with the corresponding light colours, on the horizontal axis. For an action spectrum, calculate photosynthesis rate from oxygen production or carbon-dioxide consumption at each wavelength and plot rate on the vertical axis.
If oxygen production is greatest in blue and red light and lowest in green light, the action spectrum has blue and red peaks that broadly match pigment absorption; accessory pigments can make the curves differ.
Absorption and photosynthesis rate are different dependent variables. Do not label an action-spectrum y-axis as absorbance, and compare rates only when intensity and other limiting factors are controlled.
This objective is assessed through structured response, commonly using Describe / Distinguish / Outline.
Describe / Distinguish / Outline / Predict / Explain
Build the answer around this relationship: Chlorophyll absorbs blue and red light more strongly than green light.
Representative question
Outline how plants make use of the different wavelengths of light.
Photosynthesis converts light energy into chemical energy in carbon compounds. Carbon dioxide is reduced to carbohydrate using hydrogen from water, glucose is the main stored product, and released oxygen comes from photolysis of water. Pigment evidence is tested with chromatography and Rf, spectra questions separate absorption from action, and rate/evidence questions use limiting factors, controls, and CO2 enrichment context.
