CBSE 2024 · Region 1 · Set 1 · Q29 · 4 marks

A lens is a transparent medium bounded by two surfaces, with one or both surfaces being spherical. The focal length of a lens is determined by the radii of curvature of its two surfaces and the refractive index of its medium with respect to that of the surrounding medium. The power of a lens is reciprocal of its focal length. If a number of lenses are kept in contact, the power of the combination is the algebraic sum of the powers of the individual lenses.

(i)

A double-convex lens, with each face having same radius of curvature R , is made of glass of refractive index n . Its power is :

(A)

$\displaystyle \frac{2(\mathrm{n}-1)}{\mathrm{R}}$

(B)

$\displaystyle \frac{(2 \mathrm{n}-1)}{\mathrm{R}}$

(C)

$\displaystyle \frac{(\mathrm{n}-1)}{2 \mathrm{R}}$

(D)

$\displaystyle \frac{(2 \mathrm{n}-1)}{2 \mathrm{R}}$

(ii)

A double-convex lens of power P , with each face having same radius of curvature, is cut into two equal parts perpendicular to its principal axis. The power of one part of the lens will be :

(A)

$\displaystyle 2 \mathrm{P}$

(B)

P (C) $\displaystyle 4$ P

(D)

$\displaystyle \frac{\mathrm{P}}{2}$

(iii)

The above two parts are kept in contact with each other as shown in the figure. The power of the combination will be :

(iii)

(A)

$\displaystyle \frac{\mathrm{P}}{2}$

(B)

P (C) $\displaystyle 2$ P

(D)

$\displaystyle \frac{\mathrm{P}}{4}$

(a)

A double-convex lens of power P , with each face having same radius of curvature, is cut along its principal axis. The two parts are arranged as shown in the figure. The power of the combination will be :

(a)OR(b)

(A)

Zero

(B)

P (C) $\displaystyle 2 \mathrm{P}$

(D)

$\displaystyle \frac{\mathrm{P}}{2}$

CBSE Class 12 Physics past-paper question from the 2024board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.