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A double convex thin lens made out of glass (refractive index, \mu =1.5) has both radii of curvature of magnitude \mathrm{20\ cm}. Incident light rays parallel to the axis of the lens will converge at a distance \mathrm{d\ cm} such that:

Option: 1

\mathrm{d=10}


Option: 2

\mathrm{d=20/3}


Option: 3

\mathrm{d=40}


Option: 4

\mathrm{d=20}


Answers (1)

best_answer

\mathrm{Here,\ \mathrm{f}=\mathrm{d}, \mathrm{R}_1=+20 \mathrm{~cm}, \mathrm{R}_2=-20 \mathrm{~cm}} \\ \mathrm{\frac{1}{\mathrm{f}}=(\mu-1)\left[\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right] \Rightarrow \frac{1}{\mathrm{~d}}=(1.5-1)\left[\frac{1}{20}-\frac{1}{-20}\right]}
\begin{aligned} & \frac{1}{d}=0.5 \times \frac{2}{20} \\ & \therefore \quad d=20 \mathrm{~cm} \end{aligned}

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SANGALDEEP SINGH

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