Electromagnetic radiation
[EnvThes] Photosynthetically active radiation (PAR) capable of driving light reactions of photosynthesis, wavelength 380 and 710 nm.
[Wikipedia] Photosynthetically active radiation, often abbreviated PAR, designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place.
[USDA] That portion of the electromagnetic spectrum that is the energy source utilized for photosynthesis and generally coincides with the visible light spectrum (approximately 400-700 nm). PAR is characterized by the greater absorption of the blue light and red light wavelengths by pigments.
Photosynthetically active radiation
Photons
Radiation transfer constraint