The physical law that describes the manner in which the intensity of sunlight changes as an observer moves away from the sun is the inverse square law.

Here's how it works:

* Light spreads out in all directions: Imagine the sun as a point source emitting light equally in all directions. This light spreads out like a sphere expanding outwards.

* Area increases with distance: As the distance from the sun increases, the surface area of this expanding sphere also increases.

* Intensity decreases: Since the same amount of light energy is distributed over a larger area, the intensity of light (energy per unit area) decreases.

Mathematically:

The intensity (I) of light is inversely proportional to the square of the distance (r) from the source:

* I ∝ 1/r²

Example:

If you double your distance from the sun, the intensity of sunlight will decrease to one-fourth of its original value.

Important Note:

This assumes that the sun is a point source and that there are no other factors affecting the light's intensity, such as absorption by the atmosphere.