At a distance d under the surface of the water (with refractive index n) a point source of light S is placed, the luminous intensity of which is equal to I (Fig. 145). A small platform moves along the SA line, remaining perpendicular to this line at all times (the SA line is parallel to the surface of the water). What is the minimum distance between the platform and the source at which, by calculating the illumination of the platform, the surface of the water can be considered a perfectly reflective mirror? What is the illumination of the site at this distance? Neglect the loss of light in the water column.

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