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

We are projecting a slide of 35 mm ×23 mm on a 2 m ×2 m screen using projector. Therefore, the magnification required by the projector is m= v/u (35 mm > 23 mm, so the magnification is calculated taking object size 35 mm)

m = v/u = 200/3.5

Now, from lens formula,