Which law describes how dose rate changes with distance, as used in the distance example?

• A. Inverse proportional to the square of distance
• B. Direct proportional to distance
• C. Inverse proportional to distance
• D. Direct proportional to distance squared

Answer: (A)

Dose rate from a distant X-ray source follows the inverse square law: it is inversely proportional to the square of the distance (dose rate ∝ 1/r^2). As energy radiates from a point source, it spreads over the surface of an expanding sphere, whose area grows with r^2. So doubling the distance makes the energy cover four times the area, reducing the dose rate to one quarter; tripling distance reduces it to one ninth. This is why the correct description is inverse proportional to the square of distance. The other relationships would imply the dose rate increases with distance or decreases only linearly, which doesn’t match how radiation intensity falls off with distance for a point source.