Which cardinal rule of radiation protection is mathematically defined by the inverse square law?

• A. Time
• B. Shielding
• C. Filtration
• D. Distance

Answer: (D)

The concept being tested is how exposure changes with distance from the radiation source. The inverse square law says that radiation intensity falls off with the square of the distance, meaning if you double the distance, the exposure drops to one quarter; triple the distance, it drops to one ninth, and so on. This makes increasing distance the most powerful way to reduce dose in many situations, which is why distance is identified as the cardinal rule tied to this law.

Time, shielding, and filtration are also important protective measures, but they relate to dose in different ways: time changes dose linearly with how long you’re exposed, shielding attenuates the beam with material, and filtration filters the beam to lower effective energy, rather than relying on the 1/d^2 relationship.