Amount of energy emitted by light passing through the shaded area (sphere area)
In the SI (System of International Units), the intensity, passing through a certain area, is measured in W/m 2 .
The conservation of energy of a wave dictates that its intensity should decrease as the square of the distance from the source. Suppose the wave is being emitted with the power P , by a point source. The wave intensity at a distance d from the source is the power of the source divided by the area of a spherical surface of radius d , as shown in the figure below.
Looking at this relationship, we can see that the intensity is proportional to d 2 . The further away the observer is from the source, the lower the intensity he will perceive. For this reason, we see the Sun much more intensely than other stars, which are much further away from us.
We should not confuse the light intensity that passes through a given area with the intensity of a point light source. The intensity of the wave emitted by a point source is measured in candela, which is an SI fundamental unit.
Obs .: We cannot forget that acandela(cd) is the luminous intensity emitted in one direction by a monochromatic source that emits light with a frequency of 540 x 1012hertz and that radiates in that direction with an intensity of 1/683 watt per steradian.