ABSTRACT
There are variations in irradiance— interference fringes— seen at the boundary. As the size of the aperture is reduced, we expect that the size of the illuminated region in the observation plane will also decrease. Huygens' principle can be used to visualize this situation qualitatively. Each point on the wavefront transmitted by the aperture can be considered a source of secondary spherical waves. Any general point in the source plane gives rise to a spherical wave that emits equally in all directions. For a general source distribution, the optical disturbance in the observation plane is just a weighted sum of spherical waves that originate at the various source points that comprise the source distribution. The phenomenon of the interference depends on the principle of superposition of waves. Issues of coherence are important in practical optical systems, because ideal sources of light are never actually obtained.
