I am sure you have noticed that the apparent brightness of a light source changes depending on your distance from it. For example, a car headlight can appear dimmer than a candle if the candle is right next to your eye while the car is far away.

A fundamental principle of waves is that while the total energy emitted by a source remains constant, its intensity depends on distance:

• Power (P): A light source emits energy (E) over time, giving its power output in Watts (W = J/s). Power is an intrinsic property of the source and is independent of viewing distance.
• Intensity (I): Intensity is defined as power per unit area (I = P / A), measured in W/m². Unlike total power, intensity depends directly on distance.

As light radiates outward in a spherical shell, the surface area expands (A = 4πr²). Consequently, as distance increases by a factor of r, the observed intensity decreases by a factor of r²—a relationship known as the Inverse Square Law. For instance, doubling your distance reduces the observed intensity to one-quarter (1/4) of its original value (I ∝ 1/r²).