Question

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.

Answer 1

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