Question

Out of the total solar radiations, the proportion of PAR is

• A. Less than 50%
• B. More than 80%
• C. About 70%
• D. About 60%

Answer 1

Answer: (A)

Less than 50%

The total irradiance from the sun coming at the upper boundary of the atmosphere is 1,360 Wm${}^{-2}$. This irradiance includes ultraviolet and infrared wavelengths. Of this, only about 900 Wm${}^{-2}$ reach plants because the rest is absorbed or scattered by water vapour, dust, CO${}_2$, and ozone present in the atmosphere. Of this, about half is in the infrared, roughly 5% is in the ultraviolet, and the rest has wavelengths between 400 to 700 nm. Therefore, the plants receive about 400 to 500 Wm${}^{-2}$ photosynthetically active radiation (PAR) from the sun.

Hence, the correct answer is 'Less than 50%'.