Q40. A point light source emits electromagnetic waves uniformly into free space. A detector at distance L from the source measures intensity I₀. The detector is then moved to another point on the same spherical surface centred on the source such that the angular separation between the original and new detector positions is 45°. What intensity is measured at the new location?
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Question

Q40. A point light source emits electromagnetic waves uniformly into free space. A detector at distance L from the source measures intensity I₀. The detector is then moved to another point on the same spherical surface centred on the source such that the angular separation between the original and new detector positions is 45°. What intensity is measured at the new location?

Options

G1 Physics · Accepted Answer

For an isotropic point source, the intensity at distance R is I = P ÷ (4πR²). Both detector positions lie on the same spherical surface centred on the source, so their distance from the source remains L. The 45° angular displacement changes direction but not radial distance. Therefore the intensity remains I = I₀. Correct option: B.