Question

A concave lens has focal length of 15 cm. At what distance should
the object from the lens be placed so that it forms an image at 10 cm
from the lens? Also, find the magnification produced by the lens.

Answer 1

GIVEN :
focal length =f=-15 cm after sign conventions
image distance=V=-10cm
object distance=?
by lens formula:
1/f=1/v-1/u
1/u=1/v-1/f
= -1/10+1/15
= -3+2/30
=-1/30
u=-30cm
Magnification=m=v/u
=-10/-30=1/3=0.33

Answer 2

Answer:

Explanation:

Given :-

f = - 15 cm

v = - 10 cm

To Find :-

u = ??

m = ??

Solution :-

Using 1/v - 1/u = 1/f, we get

⇒ 1/- 10 - 1/u = 1/- 15

⇒ 1/u = - 1/10 + 1/15

⇒ 1/u = - 3 + 2/30

⇒ 1/u = 1/30

⇒ u = - 30 cm

Magnification, m = v/u

⇒ m = - 10/- 30

⇒ m =  1/3

Hence, the magnification produced by the lens is 1/3.