Which statement best describes how distance affects x-ray beam intensity?

• A. It decreases with distance according to the inverse square law
• B. It increases with distance
• C. It remains constant with distance
• D. It depends on energy only

Answer: (A)

The influence of distance on x-ray beam intensity follows the inverse square law: the intensity at a point is inversely proportional to the square of the distance from the source. This means that when you double the distance, the beam intensity becomes one quarter of what it was; when you halve the distance, it becomes four times as intense. The reasoning is that the same number of photons spreads over a larger spherical surface area as distance increases, so their concentration drops with distance. In practice, the actual intensity at the receptor is also affected by attenuation through air and the patient, but the geometric fall-off with distance itself is governed by the inverse square law. The energy of the photons influences penetration and image quality, not this distance-based geometric relationship.