Magnification
- CBSE · NCERT Class 10, Ch 9 Light - Reflection and Refraction
Magnification, m, is the height of the image divided by the height of the object, and its sign tells you whether the image is upright or inverted.
If the object is h tall and the image is h' tall, then m = h'/h. It has no unit, because it is one length divided by another. A value of 2 means the image is twice as tall. A value of 1/3 means it is one-third as tall.
The sign matters as much as the size. A positive m means a virtual, upright image. A negative m means a real, inverted image. m can also be found from the distances, but the rule differs:
- For a mirror, m = -v/u.
- For a lens, m = v/u.
A small example
A convex lens forms an image at v = +30 cm of an object at u = -15 cm. Then m = v/u = (+30 cm)/(-15 cm) = -2. The image is twice the size of the object. The minus sign says it is real and inverted.
Where marks go
Question 35 of CBSE's 2026-27 Science sample paper runs magnification backwards. A concave lens gives an image one-third the size of an object 24 cm away. The 2026-27 Science marking scheme first reasons that the image is upright and smaller, so m = +1/3. Then v = mu = (1/3) × (-24 cm) = -8 cm. That line earns 1 mark.
The mark depends on choosing the plus sign for an upright image. Each sign follows the New Cartesian Sign Convention. Read how step marking works.
Sources
Facts last checked against these sources on 30 September 2026.
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