The inverse square law describes how radiation intensity changes with distance.

• A. Inversely proportional to the square of distance
• B. Directly proportional to distance squared
• C. Independent of distance
• D. Proportional to distance

Answer: (A)

Intense radiation from a point source diminishes as distance increases because the energy spreads out over a growing spherical surface. The amount of energy per unit area drops with the surface area, which grows proportional to the distance squared. So, the intensity is inversely proportional to the square of the distance. For example, doubling the distance makes the same energy cover four times as much area, reducing the intensity to one quarter; halving the distance increases intensity fourfold. This is why the statement that intensity is inversely proportional to the square of distance is the correct description. The other ideas would imply increasing intensity with distance, no dependence on distance, or a simple linear change with distance, none of which matches how energy distributes in space. In dental radiology practice, this principle helps explain why increasing the source-to-object distance reduces patient dose.