Question

The magnification of an image by a convex lens is positive only when the object is placed

• A. at its focus F
• B. between F and 2F
• C. at 2F
• D. between F and optical centre
• E. beyond 2F

Hint:

Convex lens gives positive magnification only for virtual images.

Answer 1

The Correct Option is D

Concept: Magnification in lenses
\[ m = \frac{v}{u} \] Sign convention:
• $m > 0$ → image is virtual and erect
• $m < 0$ → image is real and inverted ---

Step 1: Image formation by convex lens
• Beyond F → real, inverted image → $m < 0$
• At F → image at infinity
• Between F and optical centre → virtual, erect image → $m > 0$ ---

Step 2: Identify condition for positive magnification \[ m > 0 \Rightarrow \text{virtual image} \] This occurs only when: \[ \text{object lies between } F \text{ and optical centre} \] --- Physical Interpretation:
• Lens behaves like magnifier
• Rays diverge after lens
• Image appears upright and enlarged --- Final Answer: \[ \boxed{\text{between F and optical centre}} \]