Question

1)A concave lens has a focal lenght of 15 cm.At what distance should the object be placed from the lens so that it forms an image at 10 cm from the lens?

Answer 1

focal length = -15cm
image distance = -10cm
object distance = ?

we will find object distance with a relation,
1/v - 1/u = 1/f

1/u = 1/v - 1/f

1/u = 1/-10 - 1/ -15
1/u = -1/10 + 1/15
1/u = -15 +10/150
1/u = -5/150
1/u = -1/30

 u= -30

magnification,

m= v/u ( for lens)
m = -10/-30
m= 1/3

  (or)

m= 0.33

thus the object must be placed 30cm from the mirror to get an image 10 cm.



hope it helps!!!!

Answer 2

Focal length, f = -15 cm , since it is concave lens

Image distance, v = - 10 cm

Now image distance = u

By applying Lens formula,
1/v - 1/u = 1/f
- 1/10 - 1/u = -1/15
- 1/u = - 1/15 + 1/10
- 1/u = [- 2 + 3]/30
1/u = - 1/30
u = - 30 cm


So the object should be kept 30 cm in front of the lens

Hope This Helps You