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Question 37

A point source is kept at the center of a spherically enclosed detector. If the volume of the detector increased by 8 times, the intensity will

For a point source,

Intensity varies as

$$I\propto\frac{1}{r^2}$$

If volume of spherical detector increases 8 times,

$$V'=8V$$

For sphere,

$$V=\frac{4}{3}\pi r^3$$

so

$$r'^3=8r^3$$

$$r'=2r$$

Now intensity becomes

$$I'=\frac{1}{(2r)^2}$$

$$=\frac{1}{4}\cdot\frac{1}{r^2}$$

So

$$I'=\frac{I}{4}$$

Hence intensity becomes one-fourth of original.

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