The energy flux of sunlight reaching the surface of the earth is `1.388 xx10^(3) "W/m"^(2)` . How many photons (nearly ) per square metre are incident on the Earth per second ? Assume that the photons in the sunlight have an average wavelength of 550 nm.

Explanation

Given `P = 1.388 xx10^(3) W//m^(2), lambda = 550 nm = 550 xx 10^(-9) ` m
` h = 6.63 xx 10^(-34) J-s, c= 3 xx 10^(8)` m//s
Energy of each photon ` E = (hc)/lambda = (6.63 xx10^(-34) xx 3xx 10^(8))/(550xx10^(-9)) = 3.616 xx10^(-19)` J
No. of photons incident on the earth's surface , N = `P/E=(1.388xx10^(3))/(3.66xx10^(-19))`
` :. N = 3.838 xx 10^(21) "photons/m"^(2) -s`