To calculate how many photons are in a certain amount of energy (joules) we need to know how much energy is in one photon. Start by using two equations: Energy of a photon = Frequency * Planck's constant (6.626 * 10^(-34) J-s) Speed of light (constant 3 * 10^8 m/s) = Frequency * Wavelength Which means: frequency = Speed of Light / Wavelength So energy of a photon = (Speed of light * Planck's constant)/(Wavelength) You may have seen this equation as E = hc/<span>λ</span> We have a wavelength of 691 nm or 691 * 10^-9 meters So we can plug in all of our knowns: E = (6.626 * 10^(-34) J-s) * (3.00 * 10^8 m/s) / (691 * 10^-9 m) = 2.88 * 10^(-19) joules per photon Now we have joules per photon, and the total number of joules (0.862 joules) ,so divide joules by joules per photon, and we have the number of photons: 0.862 J/ (2.88 * 10^(-19) J/photon) = 3.00 * 10^18 photons.
