What does the inverse square law state about radiation intensity as distance from the source changes?

• A. Increases by a factor of four
• B. Decreases to one-quarter of its original value
• C. Remains unchanged
• D. Increases proportionally with distance

Answer: (B)

The inverse square law describes how radiation intensity from a point source changes with distance: energy spreads over the surface of a sphere, whose area grows with the square of the distance (area ∝ r^2). Because the total energy emitted is constant, the energy per unit area—or intensity—falls off as 1/r^2. So when you double the distance from the source, the intensity becomes 1/(2^2) = 1/4 of what it was at the original distance. This means the statement that intensity decreases to one-quarter of its original value is the correct description. If you instead moved closer or farther, the change follows the same square relationship (e.g., triple the distance → intensity becomes 1/9; halve the distance → intensity becomes four times greater). This principle explains why increasing distance is an effective way to reduce exposure in radiologic practices.