An object is placed 25 cm in front of a convex lens with a focal length of 15 cm. What is the magnification produced by the lens?

Question

Question

This question was previously asked in

RRB JE CBT 2 (Mechanical Engineering) ReExam Official Paper (Held On: 04 Jun, 2025)

Answer 1

The Correct answer is M = -1.5.

Key Points

The formula for the magnification produced by a lens is M = v/u, where v is the image distance and u is the object distance.
To calculate the image distance (v), we use the lens formula: 1/f = 1/v - 1/u.
Given values:
- Object distance (u) = -25 cm (negative sign because the object is placed on the left side of the lens).
- Focal length (f) = +15 cm (positive for a convex lens).
Using the lens formula:
- 1/v = 1/f + 1/u
- 1/v = 1/15 - 1/25
- 1/v = (5 - 3) / 75
- 1/v = 2 / 75
- v = 75 / 2 = 37.5 cm.
Now calculate the magnification:
- M = v/u
- M = 37.5 / -25
- M = -1.5.
The negative sign indicates that the image formed by the convex lens is inverted.
The magnification of -1.5 means the image size is 1.5 times smaller than the object size and inverted.