Magnification is-

Magnification is defined as the ratio of the height of the image to the height of the object [2]. In the context of microscopy, this is expressed as the ratio of the observed size of the specimen (the image size) to its actual size (the true size). Magnification represents the relative extent to which an image is enlarged compared to the original object [2]. For instance, if a specimen is viewed through a lens, the magnification (m) is calculated by dividing the size the object appears in the image by its true physical size. This linear relationship is also mathematically related to the ratio of image distance (v) to object distance (u) in both spherical mirrors and lenses [4]. Therefore, the correct formula is the observed size of the specimen divided by its actual size.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Magnification > p. 156
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Magnification > p. 143
3. [4] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Solution > p. 157