15. (a) We assume all the power results in photon production at the wavelength λ = 589 nm. Let R be the rate of photon production and E be the energy of a single photon. Then, P = RE = Rhc/λ ,where E = hf and f = c/λ are used. Here h is the Planck constant, f is the frequency of the emitted light, and λ is its wavelength. Thus, R = λP hc = (589 × 10 − 9 m)(100 W) (6 . 63 × 10 − 34 J · s)(3 . 00 × 10 8 m / s) =2 . 96 × 10 20 photon / s . (b) Let I be the photon ﬂux a distance
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