What is meant by magnification? Write it's expression. What is it's sign for the (a) real (b) virtual, image?

The ratio of length of the image to the length of object, is called linear magnification. where 'I' is the length of the image, 'O' is the length of the object, 'v' is the distance of the image, 'u' is the distance of the object. (a) For a real image, linear magnification m is negative. (b) For a virtual image, linear magnification m is positive.