Two lenses of power `+ 10 D and - 5 D` are placed in contact, (i) Calculate the focal length of the combination (ii) where should an object be held from the combination so as to obtain a virtual image of magnification `2` ?

Two lenses of power `+ 10 D and - 5 D` are placed in contact, (i) Calc

1.	Two lenses of power `+ 10 D and - 5 D` are placed in contact, (i) Calculate the focal length of the combination (ii) where should an object be held from the combination so as to obtain a virtual image of magnification `2` ?
Answer» Correct Answer - (i) 20 cm (ii) `-10 cm` (i) Here, `P_1 = + 10 D, P_2 = -5 D` `P = P_1 + P_2 = + 10 - 5 = 5 D` `F = (100)/(P) cm = (100)/(5) = 20 cm` (ii) `m = + 2`, as image is virtual From `m = (f)/(u + f)` `2 = (20)/(u + 20)` `2 = (20)/(u + 20)` `2u + 40 = 20` `u = (20 - 40)/(2) = -10 cm`.

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Two lenses of power `+ 10 D and - 5 D` are placed in contact, (i) Calculate the focal length of the combination (ii) where should an object be held from the combination so as to obtain a virtual image of magnification `2` ?

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