PYQ Vault
EbooksTutor

Lens maker's formula: focal length, refractive index or radius of curvature

This type of question, asked 7 times: 2021 Paper 7, 6 Apr 2024, 2 Apr 2025, 3 Apr 2025, 28 Jan 2026 S1, 2 Apr 2026 S2, 4 Apr 2026 S2. All 7 questions below, oldest first.

Try each one first, then open its answer.

  1. Question 1

    2021 Paper 7

    15
    The thickness at the centre of a plano-convex lens is 3 mm3\text{ }mm and the diameter is 6 cm6\text{ }cm. If the speed of light in the material of the lens is 2×108 ms−12\times10^{8}\text{ }ms^{-1}, the focal length of the lens is:

    Tap an option to check your answer.

  2. Question 2

    6 Apr 2024

    100
    For the thin convex lens, the radii of curvature are at 15 cm15\text{ }cm and 30 cm30\text{ }cm respectively. The focal length of the lens is 20 cm20\text{ }cm. The refractive index of the material is :

    Tap an option to check your answer.

  3. Question 3

    2 Apr 2025

    92
    A bi-convex lens has radius of curvature of both the surfaces same as 16 cm\frac{1}{6}\text{ }cm. If this lens is required to be replaced by another convex lens having different radii of curvatures on both sides (R1≠R2)\left( R_{1}\neq R_{2} \right), without any change in lens power then possible combination of R1R_{1} and R2R_{2} is :

    Tap an option to check your answer.

  4. Question 4

    3 Apr 2025

    18
    The radii of curvature for a thin convex lens are 10 cm and 15 cm respectively. The focal length of the lens is 12 cm . The refractive index of the lens material is.

    Tap an option to check your answer.

  5. Question 5

    28 Jan 2026 S1

    11
    The magnitudes of power of a biconvex lens (refractive index 1.5) and that of a Plano-concave lens (refractive index =1.7= 1.7 ) are same. If the curvature of plano-concave lens exactly matches with the curvature of back surface of the biconvex lens, then ratio of radius of curvature of front and back surface of the biconvex lens is ____\_\_\_\_.

    Tap an option to check your answer.

  6. Question 6

    2 Apr 2026 S2

    24
    If sunlight is focused on a paper using convex lens, it starts burning the paper in shortest time when the lens is kept at 30 cm above the paper. If the radius of curvature of the lens is 60 cm then the refractive index of the lens material is α10\frac{\alpha}{10}. The value of α\alpha is ____\_\_\_\_.

  7. Question 7

    4 Apr 2026 S2

    16
    A convex lens is made from glass material having refractive index of 1.4 with same radius of curvature on both sides. The ratio of its focal length and radius of curvature is ____\_\_\_\_ .

    Tap an option to check your answer.