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The distance between a real object and its real image is 56 CM formed by a converging lens, focal length of lenses (1) f ≤ 14 (2) f > 14 (3) f = 14 (4) f = 28 |
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Answer» nfo : the distance between a real OBJECT and its real IMAGE is 56 cm formed by converging lens. To find : focal length of lens. Solution : let focal length of converging lens is F . A/c to question, u + V = 56 cm [ because real image formed by converging lens just opposite of its object ] now using formula, 1/v - 1/u = 1/f ⇒1/(56 - u) - 1/u = 1/f ⇒1/(56 - u) - 1/-u = 1/f ⇒(u + 56 - u)/(56u - u²) = 1/f ⇒56f = 56u - u² ⇒u² - 56u + 56 f = 0Discriminant = (56)² - 4 × 56f ≥ 0⇒56 × 56 ≥ 4 × 56 f ⇒14 ≥ f Therefore the focal length would be f ≤ 14 |
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