Question

Q1. A concave lens has focal length 20cm.
At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens?
Also calculate the size of the
image formed.

Q2. A convex lens has a focal length of 30 cm. Calculate at what distance should the object be placed from the lens so that it forms an image at 60 cm on the other side of the lens?. Find the magnification produced by the lens in this case.

Answer 1

Sorry, I've not solved it by typing as it would it would be difficult to type here with fractions. But, anyway, I've done it on a paper neatly.
You can check it.
NOTE:
1) Put the value of v,u,f,I and O according to sign convention.
2) Linear magnification and magnification, both are same thing.

Answer 2

1. 1/u=1/v -1/f
          = -1/15 +1/20
          =-4+3/60
           =-1/60
u=-60cm
H=v/u *h
 =-15/-60 * 5
=+1.25 cm
__________________
2. 1/u =1/v-1/f
          =1/60 -1/30
          = 1-2/60
           =-1/60
u= -60cm
m=v/u
m=60/-60
m=-1

hope this will help you...