A photon of frequency f has the energy E = hf, where h is the Planck's constant:
h = 6.63*10^(-34) J*s .
The frequency is related to the wavelength as f = c/lambda , where c is the speed of light: c = 3*10^8 m/s^2 . So the photons in the laser pulse with the wavelength of 571 nm have frequency
f = c/lambda = (3*10^8)/(571*10^(-9)) = 5.25*10^14 s^(-1) .
Each of these photons then have the energy of E = hf = 6.63*10^(-34)*5.25*10^14 = 34.8*10^(-20) J = 3.48*10^(-19) J .
So, if the total energy of the pulse is 0.188 J, this pulse contains the number of photons equal to the total energy divided by the energy of a single photon:
0.188/(3.48*10^(-19)) = 0.054*10^19 = 54*10^16 photons.
The laser pulse of the given energy and wavelength contains
540,000,000,000,000,000 photons.
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