Question

A green leaf has a surface area of 2.50 cm2 . If solar radiation is 1050 W/m2 , how many photons strike the leaf every second? Assume three significant figures and an average wavelength of 504 nm for solar radiation.

Answer 1

We know one watt per second is one joule .

thus the energy of radiation imparted on the leaf per second is 1050J / m2.

We know 1 m² = 10⁴ cm²

Thus 10⁴ square cm is irradiated with 1050 J /s

2.5 cm² will have 0.2625 J/s

Now energy of photon wit average wavelength =504 nm is calculated as hc/wavelegth

energy of photon = [6.623x 10^-34 J sec x 3.0x10⁸m/sec] / 504x 10^-9m = 3.942 x 10^-19 J

To have 0.2625 J shining on the leaf the required number of photons = 0.2624J/ 3.942x10^-19J

=6.659x 10¹⁹ photons