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If focal length of human eye is $2 \mathrm{~cm}$, then find the focal length of contact lens, such that a combined focus system of focal length $2.5 \mathrm{~cm}$ is obtained after using contact lens.
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$-10 \mathrm{~cm}$
$\begin{aligned} & (\mathbf{}): P_{\text {net }}=P_{\text {eye }}+P_{\text {lens }} \\ & \frac{1}{f_{\text {net }}}=\frac{1}{f_{\text {eye }}}+\frac{1}{f_{\text {lens }}} \\ & \frac{1}{2.5}=\frac{1}{2}+\frac{1}{f_{\text {lens }}} \text { or } f_{\text {lens }}=-10 \mathrm{~cm}\end{aligned}$
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