Optics Question 632
Question: 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 positions 40 cm apart. The power of the lens is
[SCRA 1994]
Options:
A) $ \approx $ 3 dioptres
B) $ \approx $ 5 dioptres
C) $ \approx $ 7 diopters
D) $ \approx $ 9 diopters
Show Answer
Answer:
Correct Answer: B
Solution:
$ f=\frac{D^{2}-x^{2}}{4D} $ (Focal length by displacement method)
$ \Rightarrow f=\frac{{{(100)}^{2}}-{{(40)}^{2}}}{4\times 40}=21\ cm $
$ \therefore P=\frac{100}{f}=\frac{100}{21}\approx 5D $
