Explain why the intensity of the light in a flashlight beam decreases as the flashlight moves farther away.

Ir=Initial Intensity/Area of spread=Io4πr2

Ir∝1r2

It is seen from this expression that intensity is inversely proportional to the square of the distance. As we move away from the light source the intensity decreases at the rate of square of the distance from the source.

Brightness being the perception of intensity. more the intensity more bright the object is perceived and vice versa.