How to Find the Number of Photons in Each Beam

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An electromagnetic beam may carry billions of billions of photons each second. This high number of photons describes the numerous individual energy packets that travel along the beam. A higher emission rate of photons corresponds with a higher energy emission rate, so you can calculate the number of photons in a beam from its power rating. Another factor, the beam's frequency, also determines the beam's power rating; you can take that into account using the wave's speed and wavelength.

  • Multiply the speed of light, which is 3 x 10^8 meters per second, by 6.63 x 10^-34, which is the ratio between a photon's energy and the beam's frequency: (3 x 10^8) x (6.63 x 10^-34) = 1.989 x 10^-25.

  • Divide this answer by the wavelength of the beam. For this example, imagine a beam with a wavelength of 700 x 10^-9 meters: (1.989 x 10^-25) / (700 x 10^-9) = 2.841 x 10^-19.

  • Divide the beam's power, measured in watts or joules per second, by this answer. If, for instance, the beam works at 160W: 160 / (2.841 x 10^-19) = 5.6318 x 10^20. This is the number of photons that the beam carries each second.

  • Multiply this emission rate by the length of time time for which the beam runs. Over the course of, for instance, 15 seconds: (5.6318 x 10^20) x 15 = 8.45 x 10^21. This is the total number of photons that the beam carries.

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  • Photo Credit Jupiterimages/Photos.com/Getty Images
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