Question

The light intensity 10 [m] from a point source radiating uniformly in all directions is 1000 [W/m2]. The intensity 100 [m] from the same source is:___________

Please include formulas used and show all work thanks!

Answer 1

:Intensity is defined as the amount of energy passing through unit area per second.
1000 watt means 1000 joule of energy is radiated per second.
At a distance of 1 m, 1000 joule of energy reaches the surface area of the sphere of radius 1 m
=> intensity = energy per unit area per second
1000 = E/ (4π*10^2) W/m^2
E =12.56 * 10^5 Joules

Similarly, intensity at a distance 100 m
= 12.56 * 10^5 Joules/ (4π * 100^2) W/m^2 =10 W/m^2

Intensity at 100 meters = 10 W/m^2.