Lenses give an image of the object. The size of
the image compared with the object depends on the magnification of the lens.
The
magnification of a lens is defined as:
However if we look at the diagram and consider the triangles OO'P
and II'P we can see that the magnification is also equal to image distance/object distance or
v/u.
For a convex lens with an object distance of 15 cm and an image distance
of 7.5 cm the magnification would be 7.5/15 or +0.5, the image is real and half the size of the
object.
For a convex lens with an object distance of 8 cm and an image distance of
-24 cm the magnification is -24/8 or -3, the image is virtual and three times the size of the
object.