Wavelength of light, λ = 4000 pm = 4000 x 10^{-12} m

Energy provided, E = 1 J

Energy per photon = hc/λ

If N is the number of photons that provide 1 J energy, we can write

1 J = N x hc/λ

or, N = {1 J x λ}/{hc}

= {15 x 4000 x 10^{-12} m}/{6.626 x 10^{-34} Js x 3 x 10^{8} m/s}

= 2.012 x 10^{16}