Optical Path Difference is an extremely useful measure of the performance of an imaging optical system. If the wavefronts proceeding to a given point image are spherical, concentric, and centered at the point image for a given field of view, then the imagery will be geometrically perfect, or diffraction limited. As shown earlier, the image will then be a perfect Airy disk. This is, in effect, the reverse of our earlier example where we threw a rock into a pool of water to illustrate the wave nature of light and diffraction. If we think of the water waves traveling in reverse to where the rock entered the water, we will emulate light imaging to a point image. By definition, the wavefronts will be perfectly spherical, concentric, and centered where the rock entered the water . Rays are always Perpendicular to wavefront . 

     Two-point source interference occurs when waves from one source meet up with waves from another source. If the source of waves produces circular waves, then the circular wavefronts will meet within the medium to produce a pattern. The pattern is characterized by a collection of nodes and antinodes that lie along nearly straight lines referred to as antinodal lines and nodal lines. If the wave sources have identical frequencies, then there will be an antinodal line in the exact center of the pattern and an alternating series of nodal and antinodal lines to the left and the right of the central antinodal line.