Optics Question 677
Question: An object placed 10 cm in front of a lens has an image 20 cm behind the lens. What is the power of the lens (in dioptres)
[MP PMT 1995]
Options:
A) 1.5
B) 3.0
C) ? 15.0
D) + 15.0
Show Answer
Answer:
Correct Answer: D
Solution:
$ u=-10\ cm,\ \ v=20\ cm $
$ \frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{20}-( -\frac{1}{10} )=\frac{3}{20}\Rightarrow f=\frac{20}{3}\ cm $
Now $ P=\frac{100}{f}=\frac{100}{20/3}=+\ 15\ D $
