Question

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

Answer 1

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We have,

f = 20 cm

Object distance = -u

Image distance = v

Magnification, m = -10 (negative sign implies real image)

Now, v/(-u) = -10

=> v = 10u

Using,

1/f = 1/v – 1/u

=> 1/20 = 1/10u + 1/u

=> u = 22 cm

Thus, object distance is = -u = -22 cm

So, the slide is placed at a distance 22 cm from the pole of the lens

Answer 2

Thus it is
F = 20 cm
Thus by magnification
10 = v / u
So if lens then
1/v -10/v = 1/20
V = -180 cm