Question

A slide projector has to project a 35 mm slide (35 mm x 23 mm) on a 2m x 2m screen at a distance of 10 m from the lens. What should be the focal length of the lens in the projector ?

Answer 1

For the projector the magnification required is given by

[35 mm > 23 mm, so the magnification is calculated taking object size 35 mm]
Now, from lens formula,