Question

The distance between an object and its real inverted  imageis 20.0cm. If the magnification is 0.500, what is the focal length of the lens that is being used to form the image?

Answer 1

$f= -\frac{Md}{(1-M)^2}=-\frac{0.5\times20}{(1-0.5)^2}=-40 \ cm.$