Quantrex Quantrex AcademyJEE · NEET · NDA PYQs with solutions Open app
JEE Main 202529 JanuaryEvening Shift
Exam
JEE Main
Year
2025
Date
29 January
Shift
Evening Shift
Subject
Physics
Chapter
Ray Optics

JEE Main 2025 · 29 January · Evening Shift · Physics · Ray Optics

A convex lens made of glass (refractive index =1.5 ) has focal...

A convex lens made of glass (refractive index =1.5=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

Options

  1. A24 cm
  2. B96 cm
  3. C48 cm
  4. D72 cm

Correct answer

B. 96 cm

Step-by-step solution

18=(μℓμs−1)[1R1−1R2]124=(1.5−1)[2R]...(i)1f′=(1.51.33−1)(2R)1f′=(1.5×34−1)2R...(ii)\begin{aligned} & \frac{1}{8}=\left(\frac{\mu_{\ell}}{\mu_{\mathrm{s}}}-1\right)\left[\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right] \\ & \frac{1}{24}=(1.5-1)\left[\frac{2}{\mathrm{R}}\right] ...(i)\\ & \frac{1}{\mathrm{f}^{\prime}}=\left(\frac{1.5}{1.33}-1\right)\left(\frac{2}{\mathrm{R}}\right) \\ & \frac{1}{\mathrm{f}^{\prime}}=\left(\frac{1.5 \times 3}{4}-1\right) \frac{2}{\mathrm{R}}...(ii) \end{aligned} (i) divided by (ii) f′24=4f′=96 cm\begin{aligned} & \frac{\mathrm{f}^{\prime}}{24}=4 \\ & \mathrm{f}^{\prime}=96 \mathrm{~cm} \end{aligned}
Practice Ray Optics on Quantrex Academy →

More from Ray Optics

A double convex lens made of glass of refractive index 1.5 and radii of curvature of the… JEE Advanced 2026A beam of polychromatic light passes through a thin prism of prism angle 6^∘ . The… JEE Advanced 2026Consider two isosceles prisms 1 and 2 with prism angles A_1 and A_2 and refractive… JEE Advanced 2026A thin biconvex lens is prepared from the glass ( μ=1.5 ) both curved surfaces of which… JEE Main 2026Light ray incident along a vector AO ( AO = 2 i -3 j ) emerges out along vector OB ( OB =… JEE Main 2026Angle of minimum deviation is equal to the half of the angle of prism in an equilateral… JEE Main 2026A concave mirror of focal length 10 cm forms an image which is double the size of object… JEE Main 2026A spherical interface lens of radius R separates two media of refractive indices 1 and… JEE Main 2026 Full Ray Optics list All JEE Main PYQs