JEE Main 2018PhysicsRay OpticsHardMCQ

JEE Main 2018 — Ray Optics Question with Solution

JEE Main 2018 (15 Apr)

Question

A plano-convex lens becomes an optical system of 28 cm focal length when its plane surface is silvered and illuminated from left to right as shown in fig-A If the same lens is instead silvered on the curved surface and illuminated from another side as in fig-B, it acts as an optical system of focal length 10 cm. The refractive index of the material of the lens is:

Choose an option

Show full solutionCorrect option: A
Correct answer
A1.55

Step-by-step explanation

Case-1



1 f 1 = (μ−1) R ,f=-28 ⇒P=2p1+p2
⇒128=2μ-1R

Case-2



1f1=(μ−1)R, f2=R2 , f =−10 cm
⇒P=2P1+P2⇒110=2μ-12+2R⇒110=228+2R

2R=110-228=18280⇒R=2809 cm⇒128=2μ-12809

μ-1=59⇒μ=1+59=149=1.55.

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About this question

This is a previous-year question from JEE Main 2018, covering the Ray Optics chapter of Physics. PrepSharp catalogues every PYQ from JEE Main with a verified answer key and step-by-step solution prepared by IIT alumni — so you can search by chapter, topic or year and revise efficiently.