concept

Photosynthetically active radiation (PAR)

The 400 to 700 nm band — roughly what a human eye can see — that Earth plants can use for photosynthesis. PAR is measured in photons rather than energy, because the machinery counts individual particles of light: fixing one molecule of CO₂ takes something like 8 to 10 photons regardless of how much energy each one carries. Worth remembering that the 700 nm cutoff is not a cosmic law but a property of Earth's pigments; under a redder star the useful band could extend considerably further into the infrared.