Q.21 In a simple microscope, _______.
(A) a lens with negative power is used
(B) the focal length of the lens is less than the least distance for clear vision
(C) the focal length of the lens is greater than the least distance for clear vision
(D) magnification depends only on the focal length of the lens
A simple microscope uses a single convex lens to magnify small objects by forming a virtual, enlarged image when the object is placed within the focal length. The least distance for clear vision (D), typically 25 cm, serves as the reference for maximum magnification calculated as:
Magnification formula: M = 1 + D/f where f is the focal length of the lens.
Option Analysis
- Option (A): Incorrect. A simple microscope requires a convex lens with positive power (f > 0) to converge light and produce a magnified virtual image. A concave lens (negative power) diverges light and reduces image size.
- Option (B): Correct. The lens must have f < D (e.g., f ≈ 1–5 cm) to position the object between the lens and focal point, yielding high magnification. For example,
M = 1 + 25/5 = 6. - Option (C): Incorrect. If f > D, the object cannot form a properly magnified virtual image at distance D, since the object must lie inside f for enlargement.
- Option (D): Incorrect. Magnification depends on both f and D via
M = 1 + D/f(image at D) orM = D/f(image at infinity); it varies for different observers.
Working Principle
The simple microscope focal length less than least distance of distinct vision defines its core principle for achieving magnification. This setup uses a convex lens with a short focal length (typically < 5 cm) to view tiny objects like cells or insects at high angular magnification.
When an object is placed between the lens and its focal point (u < f), an erect, virtual image forms at D = 25 cm. Diverging rays appear to emerge from the enlarged image, increasing the angular size (β) compared to unaided vision (α).
Magnification Formula
For image at the least distance of clear vision:
M = 1 + D/f
Example: If f = 5 cm, then M = 1 + 25/5 = 6. A shorter focal length yields higher magnification, confirming f < D.
Common Exam MCQs
- Directly supports answer (B).
- Contrasts with compound microscopes, which use two lenses for higher magnification.
- Frequently asked in CSIR NET, JEE, and NEET physics sections on optical instruments.
