• # question_answer A converging beam of light forms a sharp image on a screen. A lens is placed 10 cm from the screen in the path of the beam. It was found that the screen has to be moved 8 cm further away to obtain the sharp image. Focal length of the lens is- A) 6.43 cm                        B) -2.5 cmC) - 22.5 cm                      D) 2.66 cm

Since the screen has to be moved away, the lens used should be concave.   $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$ $\Rightarrow \,\,\,\frac{1}{f}=\frac{1}{18}-\frac{1}{10}$ $f=\frac{180}{10-18}\,\,=\,\,-22.5\,cm$