A point source of monochromatic light emits a power Wo = 10 W at a wavelength R = 0.5 μ. At what maximum distance will this source be seen by a person if the eye reacts to a light flux of 60 photons per second? Pupil diameter d = 0.5 cm, Planck constant h - = 6.6·10-34 J·sec.

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