Ray Optics

JEE Main level, single correct, medium

230 of 295

Question 230 of 295

To find the focal length of a convex mirror, a student records the following data: Object pin Convex Lens Convex Mirror Image Pin 22.2 cm 32.2 cm 45.8 cm 71.2 cm The focal length of the convex lens is f1{f}_{1} and that of mirror is f2{f}_{2} . Then taking index correction to be negligibly small, f1{f}_{1} and f2{f}_{2} are close to:

Show answer and solution

Answer: (A) f1=7.8cm{f}_{1}=7.8cm f2=12.7cm{f}_{2}=12.7cm

Take f2=12.7{f}_{2}=12.7

1f=1v+1u\dfrac{1}{f}=\dfrac{1}{v}+\dfrac{1}{u}

112.7=125.4+1u\dfrac{1}{12.7}=\dfrac{1}{25.4}+\dfrac{1}{u}

112.7=125.4=1u\dfrac{1}{12.7}=\dfrac{1}{25.4}=\dfrac{1}{u}

u=25.4=v′u=25.4={v}^{'}

1f=1v−1u\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}

1f1=125.4+13.6+110\dfrac{1}{{f}_{1}}=\dfrac{1}{25.4+13.6}+\dfrac{1}{10}

1f1=139+110\dfrac{1}{{f}_{1}}=\dfrac{1}{39}+\dfrac{1}{10}

1f1=39049=7.96\dfrac{1}{{f}_{1}}=\dfrac{390}{49}=7.96

The closest answers is (1) as option (3) and (4) are not possible.

JEE Main question paper PDF

Open a shift to read the full paper on this page, or download it.

JEE Main 2025 Apr 2 Shift 1 question paper

Open Download PDF

Opening paper…

Explain it as a whiteboard videoStep-by-step, in Hinglish or English. Your first video needs no login.Solved it on paper but got it wrong?Upload a photo of your working. Mistake Doctor marks the exact line that broke.
See it move: Lens ray diagram simulationDrag the sliders and watch the numbers in this question change.Revise Ray Optics firstFormula sheet, solved examples and the traps students fall into.

More Physics practice

JEE Main Physics practice question on Ray Optics (Combination of Lens and Mirrors), with the answer and a step-by-step solution. Scan a question from your own book or paper to get a whiteboard video.