Bright Tutorials Bright Tutorials
CBSE Class 10 Science Question 60 of 60

Solved 2022 Term 1 Question Paper CBSE Class 10 Science — Question 12

Back to all questions
12
Question

Question 60

If the power of the eyepiece (L2) is 5 diopters and it forms an image at a distance of 80 cm from its optical centre, at what distance should the object be?

  1. 12 cm
  2. 16 cm
  3. 18 cm
  4. 20 cm
Answer

16 cm

Reason — Given,

Power = +5D [convex lens has positive sign]

v = -80 cm.

u = ?

From formula,

P = 100f(cm)\dfrac{100}{\text{f(cm)}}

Substituting we get,

5 = 100f\dfrac{100}{f}

f = 1005\dfrac{100}{5} = 20 cm

Using formula,

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

We get,

120\dfrac{1}{\text{20}} = 1-80\dfrac{1}{\text{-80}} - 1u\dfrac{1}{\text{u}}

1u\dfrac{1}{\text{u}} = -180\dfrac{1}{\text{80}} - 120\dfrac{1}{\text{20}}

1u\dfrac{1}{\text{u}} = −1−480\dfrac{-1-4}{\text{80}} = −580\dfrac{-5}{\text{80}}

1u\dfrac{1}{\text{u}} = −116-\dfrac{1}{\text{16}}

u = -16

Hence, distance of the object from the lens = 16 cm.