Which law describes how radiation intensity changes with distance?

• A. Avogadro's Law
• B. Ohm's Law
• C. Inverse Square Law
• D. Newton's Law of Cooling

Answer: (C)

Radiation intensity decreases with distance according to the inverse square law. A point source emits energy uniformly in all directions, so the energy spreads over the surface of an expanding sphere. The surface area of that sphere grows with the square of the radius, meaning the same amount of radiation is distributed over a larger area as distance increases. Consequently, the intensity at a point is inversely proportional to the square of the distance (I ∝ 1/d^2). For example, doubling the distance reduces intensity to one quarter, and tripling distance reduces it to one ninth. This principle underpins exposure control and shielding in radiology, since increasing distance from the source reduces exposure rapidly. The other laws listed describe unrelated phenomena (Avogadro’s for gases, Ohm’s for electrical circuits, Newton’s cooling for temperature change) and do not explain how radiation behaves with distance.