Consider a circular or aperture on which a plane wavefront is falling normally as shown in the Figure 5.4. Let the screen is placed at same distance and parallel to the aperture. Let there are four points on the screen such as P1 inside, P2 outside, P3 at boundary from inside and P4 at the boundary of circle from outside. These points have corresponding the poles as 01, 02, 03 and 0 4 on the aperture. In case of 01 sufficient number of zones are exposed so we get resultant amplitude 1/2 R1 at P1 and so uniform illumination. For 02 almost all the zones are blocked so we get resultant amplitude zero at P2 and so complete darkness. For 03 and 04 few zones are exposed partially so resultant intensities at P3 and P4 is not uniform but decreases as we go from inside to outside.
FIGURE 5.4 Rectilinear propagation of light
Thus there is uniform illumination inside the circle, complete darkness outside but in between two dotted circle the intensity is not uniform. Hence one can conclude that rectilinear propagation of light is explained on the basis of wave theory but only approximately.