If the slit in Figure 1 were truly a point source of
waves, then we expect the waves to come out equally in all directions
(as indicated in the figure). Moreover, the average total power
crossing any half-sphere of radius r (and thus area 
) must equal the total power P put out by the
source. Because the intensity gives the average power per unit time
per unit area, we must have, using Eq.2 for the intensity,
where 
is the proportionality constant from Eq. 2.
The
basic fact that we have learned the amplitude of the waves from
the slit to be inversely proportional to the distance from the
slit.
