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

Explanation

Energy flux of sunlight reaching the surface of earth , `phi=1.388xx10^3 W//m^2`
Hence, power of sunlight per square metre, `P=1.388xx10^3` W
Speed of light , c`=3xx10^8` m/s
Planck's constant , h=`6.626xx10^(-34) Js`
Average wavelength of photons present in sunlight , `lambda=550 nm`
`=550xx10^(-9) m`
Number of photons per square metre incident on earth per second =n
Hence , the equation for power can be written as :
P=nE
`therefore n=P/E=(Plambda)/(hc)`
`=(1.388xx10^3 xx 550xx 10^(-9))/(6.626xx10^(-34)xx3xx10^8)=3.84xx10^21 "photons"//m^2//s`
Therefore , every second , `3.84xx10^21` photons are incident per square metre on earth .