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JEE Main 20264 AprilEvening Shift
Exam
JEE Main
Year
2026
Date
4 April
Shift
Evening Shift
Subject
Physics
Chapter
Ray Optics

JEE Main 2026 · 4 April · Evening Shift · Physics · Ray Optics

A convex lens is made from glass material having refractive index...

A convex lens is made from glass material having refractive index of 1.41.4 with same radius of curvature on both sides. The ratio of its focal length and radius of curvature is ______.

Options

  1. A0.50.5
  2. B2.52.5
  3. C0.80.8
  4. D1.251.25

Correct answer

D. 1.251.25

Step-by-step solution

Using the lens maker's formula:

1f=(μ−1)(1R1−1R2)\dfrac{1}{f} = (\mu - 1) \left( \dfrac{1}{R_1} - \dfrac{1}{R_2} \right)

For an equiconvex lens, R1=RR_1 = R and R2=−RR_2 = -R. Given μ=1.4\mu = 1.4:

1f=(1.4−1)(1R−1−R)\dfrac{1}{f} = (1.4 - 1) \left( \dfrac{1}{R} - \dfrac{1}{-R} \right)

1f=0.4×2R\dfrac{1}{f} = 0.4 \times \dfrac{2}{R}

1f=0.8R\dfrac{1}{f} = \dfrac{0.8}{R}

fR=10.8=1.25\dfrac{f}{R} = \dfrac{1}{0.8} = 1.25

Answer: 1.251.25
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