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Two thin convex lenses of power 4D and 6D are placed co-axially at a distance of 20cm apart.find the focallength and power of the combination of lense |
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Answer» ssume that be the focal length of first convex LENS and be the focal length of second convex lens. use FORMULA, here P is the combined POWER of the optical system and are the powers of the individual lenses and 'd' is the distance.here, = 4D = 6D and d = 20cm = 0.2m so, P = 4 + 6 - 0.2 × 4 × 6 hence, P = 10 - 4.8 = 5.2D focal length , = 1/4 = 25cm = 1/6 = 50/3cmnow, 1/f = 1/25 + 3/50 - 20 × 3 /(25 × 50)= 2/50 + 3/50 - 12/250 = (10 + 15 - 12)/250 = 13/250hence, f = 250/13 = 19.23cm |
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