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.

_{2}) of lens with 10 cm focal length = $\frac{100}{10}=10D$

Let Power of the lens is P

_{1}

Total power of combination = P

_{1}+P

_{2}

5D= P

_{1}+ 10D

P

_{1 }= -5D

focal length = $-\frac{1}{5}=-0.2\mathrm{m}\phantom{\rule{0ex}{0ex}}\Rightarrow -20\mathrm{cm}$

Power of the lens is negative so the given lens is concave lens

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