A light source gives off a discrete number of photons each second, emitting its energy in the form of fixed packets. The amount of energy in each photon depends on the light's wavelength, with Planck's constant describing the relationship between the two. You can therefore calculate the light source's photon emission rate from its wavelength. The energy emission rate, the other relevant factor, is equivalent to the power of the light source.
Multiply the speed of light, which is 3 x 10^8 meters per second, by Planck's constant, which is approximately 6.63 x 10^-34 joule-seconds: (3 x 10^8) x (6.63 x 10^-34) = 1.989 x 10^-25.
Divide the answer by the light's wavelength. With a wavelength, for instance, of 600 x 10^-9 meters, which corresponds with yellow light: (1.989 x 10^-25) / (600 x 10^-9) = 3.315 x 10^-19.
Divide the light source's wattage by this answer. If it operates at 150 watts: 150 / (3.315 x 10^-19) = 4.52 x 10^20. This is the number of photons of light that the source emits each second.
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