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Unformatted text preview: virtual image of the closer object located at least 70 cm from her eyes. (b) We need a lens which produces a virtual image further from the lens than the object. A converging lens will do this if the object distance is less than the focal length of the lens. ( A converging lens also makes a virtual image, but the image is always closer to the lens than is the object.) We use the thin lens formula to calculate the focal length of the required lens, which we then invert to get the power in Diopters. Since we need the focal length in units of meters, we put everything in meters to begin with. Power (in Diopters) = 1/f = 1/o + 1/i = 1/.25m + (-)1/.7m = 4 1.4 = +2.6 D uncorrected near point desired near point 0.25m 0.7m Sue s eye...
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- Spring '08