WBJEE · Physics · Ray Optics
An object placed in front of a concave mirror at a distance of \(x\) cm from the pole gives a 3 times magnified real image. If it is moved to a distance of \((x+5) \mathrm{cm},\) the magnification of the image becomes 2 . The focal length of the mirror is
- A \(15 \mathrm{cm}\)
- B \(20 \mathrm{cm}\)
- C \(25 \mathrm{cm}\)
- D \(30 \mathrm{cm}\)
Answer & Solution
Correct Answer
(D) \(30 \mathrm{cm}\)
Step-by-step Solution
Detailed explanation
Given \(y_{1}=3 x\) and \(v_{2}=2(x+5)\) So here, Ist case \[ \begin{array}{r} \frac{1}{-x}+\frac{1}{-3 x}=\frac{1}{f} \\ \frac{1}{-(x+5)}+\frac{1}{-2(x+5)}=\frac{1}{f} \end{array} \] On solving Eqs. (i) and (ii) we get \(f=30 \mathrm{cm}\)
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