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

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.

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.