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Geometrical Optics question

2019 · Q133
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Geometrical Optics question

2019 · Q133

NEETPhysicsGeometrical OpticsMCQ+4 / −1
Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is F1. When the space between the two lenses is filled with glycerin (which has the same refractive index (m = 1.5) as that of glass) then the equivalent focal length is F2. The ratio F1 : F2 will be :
  1. A
    1 : 2
  2. B
    2 : 3
  3. C
    3 : 4
  4. D
    2 : 1
View written solutionFree

Correct answer: A

NEET 2019 Physics - Geometrical Optics Question 27 English Explanation


Equivalent focal length (in air) $${1 \over {{F_1}}} = {1 \over f} + {1 \over f} = {2 \over f}$$

When glycerin is filled inside, glycerin lens behaves like a diverging lens of focal length (–f)

$${1 \over {{F_2}}} = {1 \over f} + {1 \over f} - {1 \over f} = {1 \over f}$$

$ \therefore $ $${{{F_1}} \over {{F_2}}} = {1 \over 2}$$
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