two-light-sources-of-identical-strength-are-placed-10-m-apart-an-object-is-to-be-placed-at-a-point-p

Question

Two light sources of identical strength are placed $ 10 $ m apart. An object is to be placed at a point $ P $ on a line $ \ell $, parallel to the line joining the light sources and at a distance $ d $ meters from it (see the figure). We want to locate $ P $ on $ \ell $ so that the intensity of illumination is minimized. We need to use the fact that the intensity of illumination for a single source is directly proportional to the strength of the source and inversely proportional to the square of the distance from the source.
(a) Find an expression for the intensity $ I(x) $ at the point $ P $.
(b) If $ d = 5 $ m, use graphs of $ I(x) $ and $ I'(x) $ to show that the intensity is minimized when $ x = 5 $ m, that is, when $ P $ is at the midpoint of $ \ell $.
(c) If $ d = 10 $ m, show that the intensity (perhaps surprisingly) is not minimized at the midpoint.
(d) Somewhere between $ d = 5 $ m and $ d = 10 $ m there is a transitional value of $ d $ at which the point of minimal illumination abruptly changes. Estimate this value of $ d $ by graphical methods. Then find the exact value of $ d $.

Wen Zheng · Accepted Answer

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