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JEE Advanced 2006
Exam
JEE Advanced
Year
2006
Subject
Physics
Chapter
Ray Optics

JEE Advanced 2006 · Physics · Ray Optics

A biconvex lens of focal length f forms a circular image of radius...

A biconvex lens of focal length ff forms a circular image of radius rr of sun in focal plane. Then, which option is correct?

Options

  1. Aπr2∝f\pi r^2 \propto f
  2. Bπr2∝f2\pi r^2 \propto f^2
  3. CIf lower half part is covered by black sheet, then area of the image is equal to πr2/2\pi r^2 / 2
  4. DIf ff is doubled, intensity will increase

Correct answer

B. πr2∝f2\pi r^2 \propto f^2

Step-by-step solution

r=f \tan \theta or  or r∝f∴πr2∝f2\begin{array}{lc} \text{ or } & r \propto f \\ \therefore & \pi r^2 \propto f^2 \end{array}
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