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Geometrical Optics question

2011 · Q133
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Geometrical Optics question

2011 · Q133

NEETPhysicsGeometrical OpticsMCQ+4 / −1
A biconvex lens has a radius of curvature of magnitude 20 cm. Which one of the following options describe best the image formed of an object of height 2 cm placed 30 cm from the lens ?
  1. A
    Virtual, upright, height = 1 cm
  2. B
    Virtual, upright, height = 0.5 cm
  3. C
    Real, inverted, height = 4 cm
  4. D
    Real, inverted, height = 1 cm
View written solutionFree

Correct answer: C

Focal length of the lens

1f=(1.5−1)(120−1−20){1 \over f} = \left( {1.5 - 1} \right)\left( {{1 \over {20}} - {1 \over { - 20}}} \right)f1​=(1.5−1)(201​−−201​)

⇒\Rightarrow⇒ f = 20 cm

From lens formula

1v−1u=1f{1 \over v} - {1 \over u} = {1 \over f}v1​−u1​=f1​

⇒\Rightarrow⇒ 1v−1−30=120{1 \over v} - {1 \over { - 30}} = {1 \over {20}}v1​−−301​=201​

⇒\Rightarrow⇒ v = 60 cm

IO=m=vu{I \over O} = m = {v \over u}OI​=m=uv​ = 60−30{{60} \over { - 30}}−3060​ = -2

∴\therefore∴ I = -2 ×\times× (O) = -2 ×\times× 2 = -4 cm

So image will be real inverted and of size 4 cm.

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