Find the focal length and nature of lens which should be placed in contact with a lens of focal length 10 cm so that the power of the combination becomes 5D.

Power(P2) of lens with 10 cm focal length = $\frac{100}{10}=10D$
Let Power of the lens is P1
Total power of combination = P1 +P2
​5D=​ P1 + 10D
P= -5D
focal length =
Power of the lens is negative so the given lens is concave lens

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