KCET · Physics · Ray Optics
A convex and a concave lens separated by distance are then put in contact. The focal length of the combination
- A decreases
- B increases
- C becomes 0
- D remains the same
Answer & Solution
Correct Answer
(A) decreases
Step-by-step Solution
Detailed explanation
When two lenses are placed coaxially at a distance from each other, then equivalent focal length ( ) is given by
\[
\frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{2}}-\frac{d}{f_{1} f_{2}}
\]
When the lenses are put together.
\[
\mathrm{d}=0
\]
Hence, the focal length of the combination will decrease.
\[
\frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{2}}-\frac{d}{f_{1} f_{2}}
\]
When the lenses are put together.
\[
\mathrm{d}=0
\]
Hence, the focal length of the combination will decrease.
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