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Two lenses of power +15 and - 5D are in ...

Two lenses of power +15 and - 5D are in contact with each other forming a combination lens.(a) What is the focal length of this combination? (b) An object of size 3 cm is placed at 30 cm from this combination of lenses. Calculate the position and size of the image formed.

Text Solution

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(a) ` P = P_1 + P_2 = (15 - 5) D = 10D`
` F = 1/P = 1/10 m = 10 cm `
(b) ` 1/v - 1/u = 1/F " or " 1/v = 1/F + 1/u`
or ` 1/v = 1/10 + (1)/(-30) " or " v = 15 cm`
again ` I/O = v/u = (15)/(-30) = -1/2`
The negative sign indicates that the image is real and inverted.
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