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CBSE Class 10 Science Question 43 of 60

Solved 2022 Term 1 Question Paper CBSE Class 10 Science — Question 19

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Question

Question 43

A converging lens forms a three-times magnified image of an object, which can be taken on a screen. If the focal length of the lens is 30 cm, then the distance of the object from the lens is

  1. –55 cm
  2. –50 cm
  3. –45 cm
  4. –40 cm
Answer

–40 cm

Reason — Given, convex lens as converging lens

f = 30 cm

From formula,

m = vu\dfrac{\text{v}}{\text{u}}

-3 = vu\dfrac{\text{v}}{\text{u}} [-ve sign as real and inverted image is formed.]

v = -3u

Using formula,

1f\dfrac{1}{\text{f}} = 1v\dfrac{1}{\text{v}} - 1u\dfrac{1}{\text{u}}

We get,

130\dfrac{1}{\text{30}} = 1-3u\dfrac{1}{\text{-3u}} - 1u\dfrac{1}{\text{u}}

130\dfrac{1}{\text{30}} = −1−33u\dfrac{-1-3}{\text{3u}} = −43u\dfrac{-4}{\text{3u}}

3u = -4 x 30

u = −1203\dfrac{-120}{3} = -40 cm

Hence, distance of the object from the lens = -40 cm.