Question

The distance between two point sources of light is `24cm.` Find out where would you place a converging lens of focal length `9cm,` so that the images of both the sources are formed at the same point.
A. `6cm` from `S_(1)`
B. `15 cm` from `S_(1)`
C. `10 cm` from `S_(1)`
D. `12 cm` from `S_(1)`

Answer 1

Correct Answer - A
In this case one of the image will be real and the other virtual . Let us assume that image of `S_(1)` is real and that of `S_(2)` is virtual.
Applying `(1)/(v)-(1)/(u)=(1)/(f)`
For `S_(1):(1)/(y)+(1)/(x)=(1)/(9)` `...(1)`
for `S_(2):(1)/(y)+(1)/(24-x)=(1)/(9)` `...(2)`
Solving equation (1) and (2) , we get
`x=6 cm`

Note: This question may have following answer `6 cm` from `S_(1)` or `18 cm` from `S_(2)` and `18 cm` from `S_(1)` or `6cm` from `S_(2)`.