Final answer:
The mathematical expression is that intensity is inversely proportional to the square of the distance.
Step-by-step explanation:
The mathematical expression that describes the relationship between radiation intensity and distance is that the intensity is inversely proportional to the square of the distance. This is known as the inverse square law.
For example, if we have two stars with the same luminosity and one is twice as far away as the other, it will look four times dimmer than the closer one. If it is three times farther away, it will look nine times dimmer.