Question:
A biconvex lens forms circular image of sun of radius \(r\). Find relation with focal length \(f\).
A biconvex lens forms circular image of sun of radius \(r\). Find relation with focal length \(f\).
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Image size at focal plane $\propto$ focal length.
Updated On: Apr 23, 2026
- \(\pi r^2 \propto f\)
- \(\pi r^2 \propto f^2\)
- If half covered, area = \(\pi r^2/2\)
- If \(f\) doubled, intensity increases
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The Correct Option is B
Solution and Explanation
Concept:
Angular size constant $\Rightarrow$ linear size $\propto$ focal length.
Step 1: Image size
\[ r \propto f \]
Step 2: Area
\[ A = \pi r^2 \propto f^2 \] Conclusion: \[ \pi r^2 \propto f^2 \]
Step 1: Image size
\[ r \propto f \]
Step 2: Area
\[ A = \pi r^2 \propto f^2 \] Conclusion: \[ \pi r^2 \propto f^2 \]
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