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JIPMER 2006
Exam
JIPMER
Year
2006
Subject
Physics
Chapter
Ray Optics

JIPMER 2006 · Physics · Ray Optics

If the focal length of a lens of a camera is 5 f and that of...

If the focal length of a lens of a camera is 5f5 f and that of another is 2.5f2.5 f, what is the time of exposure for the second, if for the first one is 1200 s\frac{1}{200} \mathrm{~s} ? (where ff is focal length/unit aperture)

Options

  1. A1200\frac{1}{200}
  2. B1800\frac{1}{800}
  3. C16400\frac{1}{6400}
  4. D13200\frac{1}{3200}

Correct answer

B. 1800\frac{1}{800}

Step-by-step solution

Ratio of time of expoiusers t1t2=(f1f2)2=(5f2.5f)2⇒1/200t2=(2)2⇒t2=1800 s\begin{aligned} & \frac{t_1}{t_2}=\left(\frac{f_1}{f_2}\right)^2=\left(\frac{5 f}{2.5 f}\right)^2 \\ & \Rightarrow \quad \frac{1 / 200}{t_2}=(2)^2 \\ & \Rightarrow \quad t_2=\frac{1}{800} \mathrm{~s} \\ & \end{aligned}
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