If a spherical wave front of uniform intensity (Figure 2.27) suffers no loss in traveling from its source, the total energy (PT) at each spherical surface of total area S at distance r from the center of the source is always


where Ir is the intensity at distance r and Sr is the total area of a sphere or segment of the sphere through which the total energy PT passes.

The area of a sphere is 4πr2. Let r1 and r2 correspond to the radii of two concentric spheres containing a point source at their centers radiating a total energy PT. If there are no losses, the total energy passing through S1 corresponding to r1 is equal to that through S2 corresponding to r2. From Equation 2.104


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