Light photon physics problem!?

Question

Can someone explain to me how to answer this problem. I am a little stuck...

Light is shining perpendicularly on the surface of the earth with an intensity of 900 W/m2. Assuming all the photons in the light have the same wavelength (in vacuum) of 637 nm, determine the number of photons per second per square meter that reach the earth.

Thanks!

Answer 1

First, calculate the energy of each photon.

E/photon=h*c/λ

where h is Planck's constant, λ is wavelength, c is the speed of light

Second,

Photons/s*m^2= (photon/E)*900 W/m^2

=λ/(h*c)*900w/m^2

I got ~3 X10^21 photo/s*m^2, but you'll need to be a bit more accurate