[CBSE Delhi 2026] Case Study Based Question on Ray Optics

In an experiment with convex lens of focal length f, the screen is fixed at a distance D from the object. A student slowly moves the lens away from the object towards the screen and finds that she is able to form sharp image of the object for two positions of the lens. The distance between these two positions of the lens is d.

(i) The value of d is

(A) √D(D-4f)

(B) √D(D-2f)

(C) 2√Df

(D) √D(D-f)

(ii) Compared to the size of the object, the images formed in the two positions of the lens are respectively 

(A) reduced, enlarged 

(B) reduced, reduced

(C) enlarged, enlarged 

(D) enlarged, reduced

(iii) If the distance between object and screen is 80.00 cm and the lens forms sharp images at two positions separated by 20.00 cm., the focal length of convex lens is 

(A) 15.50 cm 

(B) 18.75 cm

(C) 20.50 cm 

(D) 22.75 cm

(iv) Consider a convex lens of focal length 15 cms. For which of the following values of object-screen distance, two positions of the object can be found to obtain sharp image on the screen ?

(A) 45 cm

(B) 50 cm

(C) 55 cm

(D) 65 cm

OR

(iv) A thin convex lens of focal length 10 cm and another thin lens of focal length 'f' are placed coaxially in contact. If the power of their combination is 10/3 D, the value of 'f' is

(A) -15 cm 

(B) -10 cm 

(C) -20 cm 

(D) -30 cm

Detailed Solution is coming soon.