What happens to beam intensity when the distance between the radiation source and the object increases?

• A. Increases
• B. Remains the same
• C. Decreases
• D. Varies depending on kVp and mAs

Answer: (C)

Beam intensity follows the inverse square law: as the distance from the source to the object increases, the intensity on the object decreases proportional to 1/d^2. When you double the distance, the intensity drops to one-quarter; triple the distance, and it drops to one-ninth. This geometric spreading means fewer photons reach the object per unit area, so receptor exposure falls unless exposure factors (mainly mAs) are increased to compensate. The effect is a geometric one and is not about kVp or mAs changing the fundamental spread; those factors adjust output, but the distance-dependent decrease remains. Therefore, beam intensity decreases with increased distance.