Question

What is the number of photons of light with a wavelength of 4000 pm that provide 1 J of energy?

Answer 1

The wavelength of light is $4000pm$ or $4000\times {10}^{-12}m$ or $4000\times {10}^{-9}m$.
The energy one photon is $\frac{hc}{\lambda }=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{4\times {10}^{-9}}=4.97\times {10}^{-17}$.
The number of photons that will provide 1 J of energy is $\frac{1J}{4.97\times {10}^{-17}}J=2.01\times {10}^{18}$.