A point light source will spreads its energy equally in all directions. Therefore if you wanted to find all of the points in space where the energy was of the same intensity you would have to draw a sphere around the source point. The bigger the radius of the sphere the greater the ‘surface’ over which the energy was spread.
The relationship between radius and sphere surface area is an inverse square relationship. That means that intensity will depend on 1/r2. If you double the distance from the source the intensity will not halve but drop to a quarter of its value, tripling the distance will make the intensity drop to a ninth and so on.
Point sources of other quantities also obey the inverse square law.
- gravitational force,
- electric field,
- light,
- sound
- electromagnetic radiation
- nuclear radiation
The key is “Are your sources point sources?”
An investigation can be completed into this.