A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of he combination, is :
A
+6.67D
B
−6.5D
C
−1.5D
D
+6.5D
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Solution
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Given,
f1=+40cm
f2=−25cm
Power, P=1f
The power of combination,
P=1f1+1f2
P=100+40cm+100−25cm
P=−1.5D
The correct option is C.
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