PrepRung
Sign inSign up free

JEE Advanced 2022 Paper 1 · Physics · Ray and Wave Optics · medium

A biconvex thin glass lens of refractive index 1.5 has both radii of curvature equal to 20 cm. A small object is placed on the axis 30 cm from the…

One or more options correct

A biconvex thin glass lens of refractive index 1.51.5 has both radii of curvature equal to 20 cm20\ \mathrm{cm}. A small object is placed on the axis 30 cm30\ \mathrm{cm} from the lens in air. Choose the correct option(s).
  1. A. A real image forms 45 cm45\ \mathrm{cm} from the lens.
  2. B. The magnification is −2-2.
  3. C. If the lens and object are placed in a liquid of refractive index 1.61.6, the lens behaves as a diverging lens.
  4. D. Covering the lower half of the lens with an opaque sheet makes only the upper half of the image disappear.
Show answer and solution

Answer: B, C

Lensmaker's formula gives 1/f=0.5×(2/20)1/f=0.5\times(2/20), so f=20 cmf=20\ \mathrm{cm}. From 1v−1u=1f\dfrac1v-\dfrac1u=\dfrac1f with u=−30u=-30: v=+60 cmv=+60\ \mathrm{cm}, so (A) is false. m=v/u=−2m=v/u=-2, (B) is true. In the liquid, 1f=(1.51.6−1)220<0\dfrac1f=\left(\dfrac{1.5}{1.6}-1\right)\dfrac{2}{20}<0, so the lens diverges: (C) true. Every point of the object still sends rays through the unblocked half, so the whole image remains, only dimmer; (D) is false.
Report a mistake

Original question written in the JEE Advanced 2022 Paper 1 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake