JEE Mains · Physics · STD 12 - 9. Ray optics and optical instruments
Distance between an object and three times magnified real image is 40 cm . The focal length of the mirror used is _________ cm .
- A \(-15 / 2\)
- B \(-10\)
- C \(-20\)
- D \(-15\)
Answer & Solution
Correct Answer
(D) \(-15\)
Step-by-step Solution
Detailed explanation
\(m=-3=\frac{v}{u}\) \(v=-3u\) \(|v|-|u|=40\) \(u=20~cm,\) \(v=60~cm\) \(\therefore \frac{1}{v}+\frac{1}{u}=\frac{1}{f}\) \(\frac{1}{-60}+\frac{1}{-20}=\frac{1}{ f }\) \(f =-15 cm\)
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