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 cm
C)- 22.5 cm
D)2.66 cm
Correct Answer:
C
Solution :
Since the screen has to be moved away, the lens used should be concave. \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\] \[\Rightarrow \,\,\,\frac{1}{f}=\frac{1}{18}-\frac{1}{10}\] \[f=\frac{180}{10-18}\,\,=\,\,-22.5\,cm\]