The decrease in radiation intensity with distance follows which physical law?

• A. Inverse Square Law
• B. Compton scattering
• C. No law
• D. Radiographic density law

Answer: (A)

The decrease in radiation intensity with distance is governed by the inverse square law. Photons from a point source spread over the surface of an expanding sphere, so the area increases with the square of the distance. Doubling the distance makes the intensity drop to one quarter; tripling makes it fall to one ninth. This geometric dilution is why distance has such a strong effect on exposure. Compton scattering describes an interaction that can change energy or direction but doesn’t define how intensity changes with distance. There isn’t a separate “no law” or “radiographic density law” to explain this effect. In practice, the inverse square law provides the best framework for understanding how source-to-image distance influences exposure and image quality.