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Question
A convex lens of focal length 30 cm is placed coaxially in contact with a concave lens of focal length 40 cm. Determine the power of the combination. Will the system be converging or diverging in nature?
Solution
We have,focal lengh of convex lens,f1= +30 cm = +0.30 m and focal length
of concave lens, f2 = - 40 cm = - 0.40m
Equivalent focal lenght, `f = 1/f^1 + 1/f^2 = 1/30 + 1/-40 = (40 - 30)/1200 = 1/120 `
∴ F = 120 cm = 1.2 m
∴ Power of the combination , `p=1/F = 1/1.2 = 0.83 D`
The focal length of the combination = 1.2 m = 120 cm
As the focal length is positive, the system will be converging in nature.
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