Using the concepto of Irradiance which is the radiant flux (power) received by a surface per unit area we have that,
[tex]I = \frac{P}{A}[/tex]
Here,
P = Power
A = Area
The propagation of the irradiance is carried out in an area proportional to that of a sphere we will then have,
[tex]I = \frac{P}{4\pi r^2}[/tex]
Irradiance at a distance of 1m from the source of light is
[tex]I = \frac{1W}{4\pi (1)^2}[/tex]
[tex]I = 0.0796W/m^2[/tex]
[tex]I = 0.08W/m^2[/tex]
Irradiance at a distance of 2m from the source of light is
[tex]I = \frac{1W}{4\pi 2^2}[/tex]
[tex]I = 0.0198W/m^2[/tex]
[tex]I = 0.02W/m^2[/tex]
