Assuming the sun is directly overhead, the surface of the earth...

Assuming the sun is directly overhead, the surface of the earth receives sunlight at $1.4 imes 10^3 \text{ W m}^{-2}$. Treat the light as monochromatic with an average wavelength of $500 \text{ nm}$, and assume no light is absorbed between the sun and the earth's surface. The distance from the sun to the earth is $1.5 \times 10^{11} \text{ m}$. Determine the number of photons falling per second on each square metre of the earth's surface directly below the sun.