The Fresnel Lens Optical Landing System (FLOLS) used to ensure safe landings on aircraft carriers consists of a series of Fresnel lenses of different colors. Each lens focuses light in a different, specific direction, and hence which light a pilot sees on approach determines whether the plane is above, below, or on the proper landing path. The basic idea behind a Fresnel lens, which has the same optical properties as an ordinary lens, is shown in FIGURE 27-30. Suppose an object is 17.1 cm behind a Fresnel lens, and that the corresponding image is a distance di = d in front of the lens. If the object is moved to a distance of 12.0 cm behind the lens, the image distance doubles to di = 2d. In the FLOLS, it is desired to have the image of the lightbulb at infinity. What object distance will give this result for this particular lens?
In figure 27-30

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Remove this glass Plano-convex lens “Flatten" remaining glass to form a Fresnel lens Remaining glass has same focal length as original lens