Optics Question 581
Question: Two lenses are placed in contact with each other and the focal length of combination is 80 cm. If the focal length of one is 20 cm, then the power of the other will be
[NCERT 1981]
Options:
A) 1.66 D
B) 4.00 D
C) -1.00 D
D) - 3.75 D
Show Answer
Answer:
Correct Answer: D
Solution:
$ \frac{1}{F}=\frac{1}{f _{1}}+\frac{1}{f _{2}} $
$ \Rightarrow \frac{1}{80}=\frac{1}{20}+\frac{1}{f _{2}} $
$ \Rightarrow f _{2}=-\frac{80}{3}cm $
Power of second lens $ P _{2}=\frac{100}{f _{2}}=\frac{100}{-80/3}=-3.75\ D $
