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Question

An eye specialist prescribes spectacles having combination of convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm. 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=+40 cm=0.4 m
(for convex lens)
f2=25 cm=0.25 m
(for concave lens)

Therefore, focal length (f) of the combination,
1f=1f1+1f2
=10.4010.25=0.250.40.40×0.25
=0.150.1=1.5 D
P=1f=1.5 D
(b) is the correct option.

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