Ray Optics

JEE Main level, numerical answer, medium

189 of 295

Question 189 of 295

A point object in air is in front of the curved surface of a plano-convex lens. The radius of curvature of the curved surface is 30cm30cm and the refractive index of the lens material is 1.5,1.5, then the focal length of the lens (in cm) is ___________.

Show answer and solution

Answer: 60

1f=(μ−1)(1R1−1R2)\dfrac{1}{f}=\left(\mu -1\right)\left(\dfrac{1}{{R}_{1}}-\dfrac{1}{{R}_{2}}\right)
R1=∞{R}_{1}=\infty
R2=−30cm{R}_{2}=-30cm
1f=(1.5−1)(1∞−1−30)\dfrac{1}{f}=\left(1.5-1\right)\left(\dfrac{1}{\infty }-\dfrac{1}{-30}\right)
1f=0.530\dfrac{1}{f}=\dfrac{0.5}{30}
f = 60cm

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 (Lens Formula, Refraction of light from curved surface), with the answer and a step-by-step solution. Scan a question from your own book or paper to get a whiteboard video.