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  4. If the radius of curvature of the curved surface of a plano-convex lens is 50 cm , its focal length is (μ = 1.5)

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Q. If the radius of curvature of the curved surface of a plano-convex lens is $50 \, cm$ , its focal length is $\left(\mu = 1.5\right)$

NTA AbhyasNTA Abhyas 2022

Solution:

From lens maker's formula,
$\frac{1}{f}=\left(n - 1\right) \, \left(\frac{1}{R_{1}} - \frac{1}{R_{2}}\right)$
Given, $n=1.5$
$R_{1}=\infty \, $ (Plane surface)
$R_{2}= \, -50 \, cm=-0.5 \, m$
$\frac{1}{f}=\left(1.5 - 1\right)\times \frac{1}{0.5}$
$=0.5\times \frac{1}{0.5}=1 \, m$