Q96 / 160 · JIPMER 2005 · Physics

The radius of the convex surface of planoconvex lens is 20 cm and the refractive index of the material of the lens is 1.5. The focal length is

  • A
    30 cm
  • B
    50 cm
  • C
    20 cm
  • Correct answer
    D
    40 cm

Solution

Using the relation for focal length of planoconvex lens $=\frac{1}{f}=(\mu-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$ (Refractive index of material of lens $\mu=1.5$, $\left.R_1=20 \mathrm{~cm}, R_2=\infty\right)$ $\begin{aligned} \frac{1}{f} & =(1.5-1)\left(\frac{1}{20}-\frac{1}{\infty}\right) \\ \frac{1}{f} & =\frac{1}{40} \\ \text { or } \quad f & =40 \mathrm{~cm}\end{aligned}$

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

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