Q131 / 180 · AIIMS 2018 (27 May) · Physics

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.

  • A
    $-1 \mathrm{~cm}$
  • B
    $-5 \mathrm{~cm}$
  • Correct answer
    C
    $-10 \mathrm{~cm}$
  • D
    $-20 \mathrm{~cm}$

Solution

$\begin{aligned} & (\mathbf{}): P_{\text {net }}=P_{\text {eye }}+P_{\text {lens }} \\ & \frac{1}{f_{\text {net }}}=\frac{1}{f_{\text {eye }}}+\frac{1}{f_{\text {lens }}} \\ & \frac{1}{2.5}=\frac{1}{2}+\frac{1}{f_{\text {lens }}} \text { or } f_{\text {lens }}=-10 \mathrm{~cm}\end{aligned}$

Hence, the correct answer is option: 'C'.

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