Question

A lens of focal length 20 cm is used to produce a ten times magnified image of a flim slide on a screen. How far must the slide be placed from the lens?

Answer 1

Focal length = f = 20 cm
object distance = - u
image distance = v
magnification = m = -10 [- sign is used because the image is real since it is obtained on screen]

we know, m=v/-u
-10 = v / -u
10 u = v

using,
1/f = 1/v - 1/-u
1/20 = 1/10u + 1/u
1/20 = 1+10 / 10u
10 u / 20 = 11
u = 22 cm

therefore, the object distance is 22 cm

now v=10x22 = 220 cm