Question

a concave lens has a focal length 25 CM at what distance should the object from the lens be placed so that if it forms an image at 20cm distance from the lens?also find the magnification produced by the lens.

Answer 1

object will be placed behind 2f2 100 cm
magnification =0.2

Answer 2

Concave Lens
u = ?
v = 20cm
f = - 25 cm

1/f = 1/v - 1/u
- 1/25 = 1/20 - 1/u
- 1/25 - 1/20 = - 1/u
-4/100 - 5/100 = - 1/u
- 9 /100 = - 1/u
100/9 = u
11.1 cm = u

So the Object should be placed at a distance of 11.1 cm

m = v/u
m = 20/-11.1
m = - 1.8

Magnification is - 1.8