COMEDK · Physics · 25. Ray Optics
A convex lens of focal length \(F\) is placed in contact with a concave lens of focal length \(F\). The equivalent focal length of the combination is
- A infinity
- B \(F/2\)
- C 2 F
- D zero
Answer & Solution
Correct Answer
(A) infinity
Step-by-step Solution
Detailed explanation
Given, \(f_{1}=F\) and \(f_{2}=-F\) The equivalent focal length of combination of two lenses is \[ \frac{1}{f}=\frac{1}{f_{1}}+\frac{1}{f_{2}}=\frac{1}{F}+\frac{1}{-F}=0 \] or \[ f=\frac{1}{0}=\infty \quad \text{(infinity)} \]
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