Which statement describes the inverse square law as it applies to radiography, and its implication when changing SID?

• A. Intensity ∝ d^2; doubling SID doubles exposure.
• B. Intensity ∝ 1/d^2; doubling SID reduces exposure by two.
• C. Intensity ∝ 1/d^2; doubling SID reduces exposure by factor of four; compensation needed via higher mA or longer time.
• D. Intensity ∝ d; increasing SID increases exposure linearly.

Answer: (C)

The important idea is how intensity at the image receptor changes with distance. The inverse square law states that intensity is inversely proportional to the square of the source-to-image distance. So when you double the SID, the exposure at the receptor drops to one quarter of its previous value. To keep the same receptor exposure after increasing SID, you must increase the tube output by the same factor, typically by raising the mA or exposure time (mAs) by four. This shows why exposure does not change linearly with distance—it's a squared relationship. The other options describe linear or direct proportionalities, which don’t fit the inverse square behavior.