Under what condition does the inverse square law apply for external exposure?

• A. When the source is large relative to distance.
• B. For a point isotropic source in free space, distance is large relative to source dimensions.
• C. Only at long distances in dense media.
• D. Only for particle radiation.

Answer: (B)

Geometric spreading in free space causes external dose or flux to fall off with the square of distance, but only when the source acts as a point-like, isotropic emitter and the distance is large compared to the source size. In that far field, radiation radiates outward over a sphere with area proportional to r^2, so the intensity scales as 1/r^2 (doubling distance cuts dose rate to one-fourth). If the source is extended (not point-like) or the radiation travels through a medium that absorbs or scatters it, this simple 1/r^2 relationship no longer strictly applies. The law also isn’t limited to particle radiation; electromagnetic radiation from a small, isotropic source in free space follows the same 1/r^2 behavior in the far field.