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JEE Mains · Physics · STD 12 - Ray optics and optical instruments
  • JEE Mains 2015
  • Hard
  • Numerical

A telescope has an objective lens of focal length \(150\,\,cm\) and an eyepiece of focal length \(5 \,\,cm.\) If a \(50\,\,m\) tall tower at a distance of \(1\,\,km\) is observed through this telescope in normal setting, the angle formed by the image of the tower is \(\theta \), then \(\theta \) is close to.....\(^o\)

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Answer & Solution

Correct Answer

(A) \(30\)

Step-by-step Solution

Detailed explanation

Magnifying power of telescope,

\(M P=\frac{\tan \beta( \text { angle subtended by image at eye piece) } }{\tan \alpha \text { (angle subtended by object on objective) }}\)

Also,  \(M P =\frac{f_{o}}{f_{e}}=\frac{150}{5}=30\)

\(\tan \alpha =\frac{50}{1000}=\frac{1}{20}\, \mathrm{rad}\)

\(\therefore \quad \tan \beta=\theta=M P \times \tan \alpha\)

\(=30 \times \frac{1}{20}=\frac{3}{2}=1.5\)

or, closest value of \(\beta \approx 60^o\)

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