2 lenses in contact of power 3D and -5D form a composite lens. An object is placed around a distance 50 cm from this composite lens,find the position of the image.

Answer 1

The power of the lens combination
=P1+P2=3-5=-2D
The focal length of lens=1/p
= -100/2=-50 cm
We know the lens formula,1/f=1/v-1/u
now u=-50 cm
-1/50=1/v+1/150
Now solve it....
After you will get -25 as v.
Therefore,position of image is 25 cm from the composite lens.

Answer 2

Here
F= 1/p =1/-2
But u have gave it as 100/2
How it is possible?