Question

A converging beam of rays in incident on a diverging lens. Having passed through the lens the rays intersect at a point `15 cm` from the lens. If the lens is removed, the point where the rays meet, move `5 cm` closer to the mounting that holds the lens. Find the focal length of the lens.
A. `f=10cm`
B. `f=15cm`
C. `f=30cm`
D. `f=40cm`

Answer 1

Correct Answer - c.
`(1)/(v)-(1)/(u)=(1)/(f)` Here, `v=15 cm` and `u=10 cm`
Hence, `(1)/(f)=(1)/(15)-(1)/(10)=-(1)/(30)cm`
`|f|=30cm`