The distance-exposure relationship is described by which law?

• A. Inverse proportion
• B. Inverse square law
• C. Direct proportion
• D. Quadratic relation

Answer: (B)

The distance-exposure relationship follows the inverse square law. Picture the X-ray beam as radiating from a point source and spreading in all directions, so the energy that reaches a target is spread over the surface area of a sphere. That area grows with the square of the distance, so the exposure at the receptor is proportional to 1/d^2. In practical terms, doubling the distance reduces exposure to one quarter, while halving the distance increases exposure fourfold. This principle is why changes in source-to-image distance (SID) have such a strong effect on image exposure. The other relationships don’t fit this behavior: inverse proportion would drop off too slowly (1/d), direct proportion would mean exposure increases with distance, and a quadratic increase with distance would imply more exposure as you move farther away, which doesn’t match how diverging beams distribute energy.