Why does the brightness of the light emitted by a flashlight decrease as the flashlight moves farther away from you?

1 Answer
Feb 6, 2016

Due to inverse square law followed by light.

Explanation:

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Let a light source of intensity Io be placed at the center of a sphere of radius r. The light travels and spreads equally in all directions.
The surface area of the sphere is given by the expression 4πr2.
As such the intensity of light at a distance equal to the radius of the sphere is given by
Ir=Initial Intensity/Area of spread=Io4πr2
Ir1r2

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, i.e., more the intensity more bright the object is perceived and vice versa.