The energy flux of sunlight reaching the surface of the earth is `1.388 xx 10^(3) W .m^(-2)`. How many photons (nearly) per square metre are incident on the earth per second? Assumethat the photons in the sunlight have an average wavelength of 550 nm.

Explanation

Wavelength `lamda = 550 nm = 550 xx 10^(-9)m`
Energy flux, `phi = 1.388 xx 10^(3) W.m^(-2)`
Energy of each photon,
`E = (hc)/(lamda) = (6.626 xx 10^(-24) xx 3 xx 10^(8))/(550 xx 10^(-9)) = 3.61 xx 10^(-19)J`
`:.` Number of photons incident on earth per square metre per second,
`n = (phi)/(E) = (1.388 xx 10^(3))/(3.61 xx 10^(-19)) = 3.85 xx 10^(21)`