Optics Question 580
Question: A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of combination is
[IIT-JEE 1982; AFMC 1997; CBSE PMT 2000; RPMT 2003]
Options:
A) - 1.5 D
B) - 6.5 D
C) + 6.5 D
D) + 6.67 D
Show Answer
Answer:
Correct Answer: A
Solution:
Focal length of the combination can be calculated as $ \frac{1}{F}=\frac{1}{f _{1}}+\frac{1}{f _{2}} $
$ \Rightarrow \frac{1}{F}=\frac{1}{(+40)}+\frac{1}{(-25)} $
$ \Rightarrow F=-\frac{200}{3}cm $
$ \therefore P=\frac{100}{F}=\frac{100}{-200/3}=-1.5\ D $
