Photons travel in electromagnetic beams, oscillating through a plane at right angles to the wave's direction. The wavelength is the distance between peaks of the wave, and this value is related to the number of photons that the beam carries. A wave with a lower wavelength carries more energy, and this energy increase corresponds with a greater number of photons moving each second. The ratio that relates the energy in each photon to the wavelength is Plank's constant, which is approximately 6.63 x 10^-34 joule-seconds.
Divide the beam's power, measured in Watts, by the number of photons that it carries each second. For this example, imagine a beam that works at 200W and carries 8.45 x 10^21 photons each second -- 200 / (8.45 x 10^21) = 2.367 x 10^-20.
Divide Planck's constant by this answer -- (6.63 x 10^-34) / (2.367 x 10^-20) = 2.801 x 10^-14.
Multiply your answer by the speed of light, which is 3.0 x 10^8 meters per second -- (2.801 x 10^-14) x (3.0 x 10^8) = 8.403 x 10^-6 meters. This is the wavelength of the photons' wave.
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