Q. The distance between an object and the screen is $100\ cm$. A lens produces an image on the screen when placed at either of the position $40\ cm$ apart. The power of the lens is
A) $\approx 3\ dioptres$
B) $\approx 5\ dioptres$
C) $\approx 7\ dioptres$
D) $\approx 9\ dioptres$
Explanation
(b) $f = \frac{D^{2} - x^{2}}{4D}$ [Focal length by displacement method]
$\Rightarrow f = \frac{(100)^{2} - (40)^{2}}{4 \times 100} = 21\ cm$
$\therefore P = \frac{100}{f} = \frac{100}{21} = 5D$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.