Final answer:
PAR (Photosynthetically Active Radiation) spans the major wavelengths of light (400-700 nm) used by plants for photosynthesis. It aligns closely with the visible light range for humans, but it is a term used in plant physiology rather than describing photon rates.
Step-by-step explanation:
The irradiation or light measurement known as PAR (Photosynthetically Active Radiation) refers to the range of wavelengths from 400-700 nm. This measure is specifically relevant to photosynthesis, as it spans the major wavelengths of light that plants use to convert light energy into chemical energy. PAR does not refer to the photon aspect rate, nor does it define the photon assimilation rate of only red and blue light. Instead, it encompasses a broader range that includes all the colors of light that are used by plants for photosynthesis. While it is closely aligned with the range of visible light for humans, which generally lies within 380 to 740 nm, PAR is a term typically used in the context of plant physiology.
It is essential to note that PAR is an important concept in understanding how plants harness energy from light. To further clarify the options given in the question: 'd. spans the major wavelengths of light that are used to drive photosynthesis' is the correct definition for PAR within the range of 400-700 nm. This range does align closely with visible light for mammals, which includes humans, though slight variations may exist among different species. The focus on photons in options 'a' and 'c' is misleading because PAR does not describe photon rates, but rather the waveband within which the energy for photosynthesis is captured.