Optics Question 597

Question: Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact. The combined focal length is

[CPMT 1973, 89; BVP 2003]

Options:

A) + 15 cm

B) -15 cm

C) + 30 cm

D) -30 cm

Show Answer

Answer:

Correct Answer: D

Solution:

$ \frac{1}{F}=\frac{1}{f _{1}}+\frac{1}{f _{2}}=\frac{1}{60}+\frac{1}{(-20)}\Rightarrow F=-30 $



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