A) \[f\,\text{and}\frac{1}{4}\]
B) \[\frac{3f}{4}\text{and}\frac{I}{2}\]
C) \[f\,\text{and}\,\frac{3I}{4}\]
D) \[\,\frac{f}{2}\,\text{and}\,\frac{I}{2}\]
Correct Answer: C
Solution :
Intensity, \[I\propto {{A}^{2}}\] \[\Rightarrow \]\[\frac{{{I}_{2}}}{{{I}_{1}}}={{\left[ \frac{{{A}_{2}}}{{{A}_{1}}} \right]}^{2}}=\frac{{{\pi }^{2}}-\frac{\pi {{r}^{2}}}{4}}{\pi {{r}^{2}}}=\frac{3}{4}\] \[\Rightarrow \]\[{{I}_{2}}=\frac{3}{4}{{I}_{1}}\]and focal length remains unchanged.
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