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