The magnitudes of power of a biconvex lens and a plano concave lens are same
Q. The magnitudes of power of a biconvex lens (refractive index 1.5) and that of a plano-concave lens (refractive index = 1.7) are same. If the curvature of plano-concave lens exactly matches with the curvature of back surface of the biconvex lens, then ratio of radius of curvature of front and back surface of the biconvex lens is ______.

(A) 2 : 5

(B) 5 : 2

(C) 12 : 5

(D) 5 : 12

Correct Answer: 5 : 2

Explanation

Using the lens maker formula:

P = (μ − 1) ( 1/R1 − 1/R2 )

For the biconvex lens (μ = 1.5):

P = 0.5 ( 1/Rf + 1/Rb )

For the plano-concave lens (μ = 1.7):

P = 0.7 ( 1/Rb )

Given that magnitudes of powers are same:

0.5 ( 1/Rf + 1/Rb ) = 0.7 ( 1/Rb )

Solving:

1/Rf = (1.4 − 1)/Rb = 0.4 / Rb

Rf / Rb = 5 / 2

Hence, the required ratio is:

5 : 2

Related JEE Main Questions

Two point charges of 1 nC and 2 nC are placed at the two corners of equilateral triangle of side 3 cm. The work done in bringing a charge of 3 nC from infinity to the third corner of the triangle is ____ μJ.
A block of mass 5 kg is moving on an inclined plane which makes an angle of 30° with the horizontal. Friction coefficient between the block and inclined plane surface is √3/2. The force to be applied on the block so that the block will move down without acceleration is ____ N.
For the two cells having same EMF E and internal resistance r, the current passing through the external resistor 6Ω is same when both the cells are connected either in parallel or in series. The value of internal resistance r is _____ Ω.
Given below are two statements : Statement I : A plane wave after passing through prism remains as plane wave but passing through small pin hole may become spherical wave. Statement II : The curvature of a spherical wave emerging from a slit will increase for increasing slit width.
The magnetic field at the centre of a current carrying circular loop of radius R is 16 μT. The magnetic field at a distance x = √3 R on its axis from the centre is ______ μT.
JEE Main Chapter-wise Previous Year Questions

Updated for JEE Main 2026: This PYQ is important for JEE Mains, JEE Advanced and other competitive exams. Practice more questions from this chapter.

Scroll to Top