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.

Options

A. `- 10 cm`
B. 20 cm
C. `- 30 cm`
D. 5 cm

Answer

C

Explanation

Here, `O` acts as a virtual object and `I` acts as final image of the converging beam of light passing through a diverging lens.
Here, `u = + 10 cm, v = + 15 cm, f = ?`
As `(1)/(f)=-(1)/(u)+(1)/(v) :. (1)/(f)=-(1)/(10) + (1)/(15) = -(1)/(30)`
or `f = - 30 cm`.
.