Inverse Square Rule

• The inverse square rule states that the intensity of a physical phenomenon, like light or gravity, is inversely proportional to the square of its distance from the source.

  $I =$$\frac{1}{r^{2}}$

  e.g., doubling the distance reduces intensity to one-quarter.
• The image shows how light from a source (S) spreads out over three surfaces at distances$r$,$2r$, and$3r$. Notice how the same amount of light is spread over increasingly larger areas, resulting in a decrease in light intensity (brightness) at greater distances. The letter 'A' represents a section of each surface of equal size – at$3r$, the same amount of light is spread over a much larger area.