Question

The condenser lens of a slide projector has a focal length of 25 mm. It produces a twice magnified image of the lamp filament. How far should the projection lens be from the condenser?

Ans.: 75 mm

Answer 1

Here ,

for the magnification

m = -v/u

-2 = v/u

v = -2 u

Now, for the lens formula

1/f = 1/v + 1/u

1/25 = 1/2u + 1/u

solving for u

u = 75 mm

the object distance is 75 mm