Here's why:

* Inverse Square Law: Illuminance (the amount of light falling on a surface) is inversely proportional to the square of the distance from the light source. This means:

* If you double the distance, the illuminance drops to one-fourth.

* If you triple the distance, the illuminance drops to one-ninth.

* Spread of Light: As light travels away from the source, it spreads out over a larger area. This means the same amount of light is distributed over a wider surface, resulting in lower illuminance.

Think of it like this: Imagine a flashlight shining on a wall. If you move the flashlight further away, the light spot on the wall becomes larger and less bright.

In summary: The further you move away from a light source, the less light falls on a given surface, resulting in lower illuminance.