Think about a point source which is producing radiation in all directions. The radiation spreads out as it moves away from the source so if you measure the intensity at a given distance away (say 10 cm), the value you get is total emission rate spread over the surface of a sphere with radius 10 cm.
If you now measure the intensity 20 cm away, the value you measure will be the same total emission rate, but now spread more thinly over the surface of a sphere with 20 cm radius
The surface area of a sphere is related to the square of the radius (4 pi x radius^2), so if you double the radius, you increase the surface area by four. So, by doubling the radius, you have spread your total emission rate over 4x the surface area; in other words, the measured intensity will have decreased by 4x.
Hope this helps
C
from the point, say 10 cm, the