A converging beam of light forms a sharp image on a screen. A lens is placed in the path of the beam at `10 cm` from the screen. It is found that the screen has to be moved `8 cm` further away from the lens to obtain a sharp image. Find the focal length and nature of the lens.

Explanation

The image is formed on screen i.e., image is real.Due to lens. Screen is shifted away i.e., lens is diverging. Here object is virtual and image is real.
`u=+10cm,v=+18cm,f=?`
`(1)/(f)-(1)/(v)-(1)/(u)=(1)/(18)-(1)/(10)=(5-9)/(90)=-(4)/(90)`
`f=-22.5`cm