When we see an image produced by a mirror or a lens, then it may be larger, smaller or equal in size with respect to that of the object.
Magnification is used to measure how large or small the size of the image is compared to that of the object formed in a mirror or a lens.
In other words, the ratio of the length of the image of that object is called linear magnification or in short magnification.
If an image of length l’ is formed in a mirror or a lens for an object of length 1, then magnification of the object is the ratio of 1‘ to l
Therefore, m = 1‘ / l
From the magnitude of magnification m, we know how many times larger or smaller the image is compared to that of the object.