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 $d$.

Year	Paper Code	Set	Position
CBSE 2026	Series 55-2	Set 1	Question 30
CBSE 2026	Series 55-2	Set 2	Question 29
CBSE 2026	Series 55-2	Set 3	Question 30

• Step 1: Stating the fundamental formula / law statement (+1 Mark)
• Step 2: Intermediate mathematical substitution or proof steps (+1 Mark)
• Step 3: Correct final value or conclusion with mandatory SI units (+1 Mark)