*PLEASE HELP* When an object is placed in front
of a convex lens, it creates a virtual
image at -12.8 cm with a
magnification of 2.85. What is the
focal length of the lens?
(Mind your minus signs.)
(Unit = cm)

The focal length of the lens = 3.32 cm.

Explanation:

Lens formula :    (i)

f= focal length , v=image distance , u =object distance.

magnification: m =   (ii )

Given: v= -12.8 cm , m =2.85

Put values in (ii), we get

substitute values of u , v in (i)

Hence, the focal length of the lens = 3.32 cm.

conservation of mass

mass before something happens = mass afterwards