PAR range is :

Question

PAR range is :

Accepted Answer

The correct option is 2: 400 nm 700 nm

Photosynthetically Active Radiation, or PAR, refers to the segment of the electromagnetic spectrum that photosynthetic organisms—such as plants, algae, and cyanobacteria—can actively use for the process of photosynthesis. This range, scientifically defined, corresponds almost exactly to the human visible spectrum, spanning from **400 nanometers (nm) to 700 nanometers (nm)**. This particular range is crucial because it aligns precisely with the absorption maxima of the major photosynthetic pigments, especially chlorophylls $a$ and $b$, and carotenoids. Chlorophyll strongly absorbs light at the blue end (around 430 nm) and the red end (around 662 nm) of this spectrum, while reflecting green light, which is why plants appear green. Therefore, the solar radiation within the 400–700 nm band is the energy source that powers the chemical reactions converting carbon dioxide and water into glucose, making it the fundamental range for all primary production on Earth.

The incorrect option is 1: 200 nm-800 nm

This range is incorrect because it is significantly too broad and includes wavelengths that are either harmful or photosynthetically inefficient. The 200 nm to 400 nm region, known as ultraviolet (UV) radiation (specifically UVC and UVB), carries enough energy to damage DNA and proteins, and is generally avoided or filtered by plants rather than utilized for photosynthesis. Similarly, the segment from 700 nm to 800 nm falls into the near-infrared region. While some far-red light (up to 750 nm) can be used, particularly in combination with shorter wavelengths, this region as a whole is not classified as PAR and is primarily involved in photomorphogenesis (plant development) rather than core photosynthesis.

The incorrect option is 3: 400 µm - 700 µm

This option is incorrect due to the use of an inappropriate unit of measurement. The symbol 'µm' stands for micrometer, which is $10^{-6}$ meters. If the range were 400 µm to 700 µm, this would correspond to 400,000 nm to 700,000 nm. Light in this wavelength band falls into the far-infrared or submillimeter wave regions of the electromagnetic spectrum, which is primarily experienced as heat and has zero energy for driving the photochemical reactions of photosynthesis. For visible and photosynthetically active light, the nanometer (nm), which is $10^{-9}$ meters, is the correct and standard scientific unit to express the wavelength.

The incorrect option is 4: 200 µm - 800 µm

Like option 3, this option is incorrect because it uses micrometers (µm) instead of nanometers (nm). Expressing the PAR range in micrometers results in a value that lies in the far-infrared/terahertz spectrum. Light with wavelengths in the hundreds of micrometers has extremely low energy per photon and is completely incapable of exciting the electrons in chlorophyll molecules, which is the necessary first step for photosynthesis. The biological process of photosynthesis relies on the high energy of photons within the visible (400–700 nm) light spectrum, thus confirming that both the numerical range and the unit in option 2 are uniquely correct.