JEE Main · 2026 · 24 Jan 2026 · Shift 2 · Waves & Sound

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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

(A)increase by 8 times
(B)increase by 64 times
(C)decrease by 8 times
(D)decrease by 4 times✓ Correct

Answer: (D)

Solution

The source emits a fixed power

P

spread over the enclosing sphere, so the intensity at the wall is

I = \frac{P}{4 π R ^{2}}

. Multiplying the volume by

8

multiplies the radius by

38 = 2

, so

R \to 2 R

and the area grows by

4

. The intensity therefore falls to

\frac{I}{4}

, i.e. it decreases by

4

times.

JEE Main · 2026 · 24 Jan 2026 · Shift 2 · Single correct option

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