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Geometrical Optics question

2000 · Q135
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Geometrical Optics question

2000 · Q135

NEETPhysicsGeometrical OpticsMCQ+4 / −1
For a plano convex lens (μ\muμ = 1.5) has radius of curvature 10 cm. It is silvered on its plane surface. Find focal length after silvering
  1. A
    10 cm
  2. B
    20 cm
  3. C
    15 cm
  4. D
    25 cm
View written solutionFree

Correct answer: A

1f=(μ−1)(1R1−1R2){1 \over f} = \left( {\mu - 1} \right)\left( {{1 \over {{R_1}}} - {1 \over {{R_2}}}} \right)f1​=(μ−1)(R1​1​−R2​1​)

= (1.5−1)(1∞−1−10)\left( {1.5 - 1} \right)\left( {{1 \over \infty } - {1 \over { - 10}}} \right)(1.5−1)(∞1​−−101​) = 0.5(110)0.5\left( {{1 \over {10}}} \right)0.5(101​)

⇒\Rightarrow⇒ f = 20 cm

When plane surface is silvered, F = f2=202{f \over 2} = {{20} \over 2}2f​=220​ = 10 cm

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