WBJEE · Physics · Ray Optics
An object is placed \(30 \mathrm{cm}\) away from a convex lens of focal length \(10 \mathrm{cm}\) and a sharp image is formed on a screen. Now a concave lens is placed in contact with the convex lens. The screen now has to be moved by \(45 \mathrm{cm}\) to get \(\underline{a}\) sharp image again. The magnitude of focal length of the concave lens is (in \(\mathrm{cm}\) )
- A 72
- B 60
- C 36
- D 20
Answer & Solution
Correct Answer
(D) 20
Step-by-step Solution
Detailed explanation
For the first condition \[ \frac{1}{f_{1}}=\frac{1}{v}-\frac{1}{u} \Rightarrow \frac{1}{10}=\frac{1}{v}+\frac{1}{30} \] \(\frac{1}{v}=\frac{1}{10}-\frac{1}{30}\) \(v=\frac{30}{2}=15 \mathrm{cm}\)…
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