An eye specialist prescribes spectacles having combination of convex lens of focal length 40cm in contact with a concave lens of focal length 25cm. The power of this lens combination in diopters is
A
+1.5
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B
−1.5
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C
+6.67
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D
−6.67
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Solution
The correct option is B−1.5 f1=+40cm=0.4m
(for convex lens) f2=−25cm=−0.25m
(for concave lens)
Therefore, focal length (f) of the combination, 1f=1f1+1f2 =10.40−10.25=0.25−0.40.40×0.25 =−0.150.1=−1.5D ⇒P=1f=−1.5D
(b) is the correct option.