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Focal length or power of a lens immersed in a liquid

This type of question, asked 9 times: 2021 Paper 6, 28 Jul 2022, 24 Jan 2023, 11 Apr 2023, 29 Jan 2024, 24 Jan 2025, 29 Jan 2025, 7 Apr 2025, 28 Jan 2026 S1. All 9 questions below, oldest first.

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  1. Question 1

    2021 Paper 6

    11
    The refractive index of a converging lens is 1.4. What will be the focal length of this lens if it is placed in a medium of same refractive index? (Assume the radii of curvature of the faces of lens are R1R_{1} and R2R_{2} respectively)

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  2. Question 2

    28 Jul 2022

    106
    The power of a lens (biconvex) is 1.25 m−11.25{\text{ }m}^{- 1} in particular medium. Refractive index of the lens is 1.5 and radii of curvature are 20 cm20\text{ }cm and 40 cm40\text{ }cm respectively. The refractive index of surrounding medium:

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  3. Question 3

    24 Jan 2023

    112
    A convex lens of refractive index 1.5 and focal length 18 cm18\text{ }cm in air is immersed in water. The change in focal length of the lens will be ____\_\_\_\_ cmcm (Given refractive index of water =43=\frac{4}{3} )

  4. Question 4

    11 Apr 2023

    22
    The radius of curvature of each surface of a convex lens having refractive index 1.8 is 20 cm20\text{ }cm. The lens is now immersed in a liquid of refractive index 1.5. The ratio of power of lens in air to its power in the liquid will be x:1x:1. The value of xx is___

  5. Question 5

    29 Jan 2024

    15
    A biconvex lens of refractive index 1.5 has a focal length of 20 cm20\text{ }cm in air. Its focal length when immersed in a liquid of refractive index 1.6 will be:

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  6. Question 6

    24 Jan 2025

    15
    A plano-convex lens having radius of curvature of first surface 2 cm exhibits focal length of f1f_{1} in air. Another plano-convex lens with first surface radius of curvature 3 cm has focal length of f2f_{2} when it is immersed in a liquid of refractive index 1.2. If both the lenses are made of same glass of refractive index 1.5 , the ratio of f1f_{1} and f2f_{2} will be :-

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  7. Question 7

    29 Jan 2025

    88
    A convex lens made of glass (refractive index == 1.5) has focal length 24 cm in air. When it is totally immersed in water (refractive index =1.33= 1.33 ), its focal length changes to

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  8. Question 8

    7 Apr 2025

    8
    A lens having refractive index 1.6 has focal length of 12 cm , when it is in air. Find the focal length of the lens when it is placed in water. (Take refractive index of water as 1.28)

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  9. Question 9

    28 Jan 2026 S1

    24
    A convex lens of refractive index 1.5 and focal length f=18 cmf = 18\text{ }cm is immersed in water. The difference in focal lengths of the given lens when it is in water and in air is α×f\alpha \times f. The value of α\alpha is ____\_\_\_\_ . (refractive index of water =4/3= 4/3 )