How many photons are contained in a flash of green light (525 nm) that contains 189 kJ of energy?
c=f w c=speed of light (m/s) f= frequency (Hz) w=wavelength (m) (eqn 1)
E=h f E= energy (J) h= planck’s constant (6.626e-34 J*s) (eqn 2)
Substitute eqn 1 into eqn 2 using frequency
E=(hc)/w (6.626e-34*3e8)/ 535e-9 = 3.72e-19 J/photon
3.72e-19 Divide by 1000 to get kJ per photon = 3.72e-22 kJ/photon
You have 189 kJ of energy, divide by 3.72e-22 kJ/photon, the kJ term cancels and your left with # of photons