It is actually not a homework, but rather a paper I need to present, talking about the advance attempts to raise microscopy axial resolution. It is said the axial resolution is usually 3 times lower than lateral. The explanation is that the lens from 1 side of focus plane can only produce a segment of spherical wavefront, and thus, making the focal spot longer than wide. (If there were a whole spherical wavefront, the focal spot would be spherical too.) People also have tried to get close to this spherical wavefront by introducing another lens at the opposite side of the focus plan, thus reduce the length of the focal spot. (4Pi Fluorescence microscopy - invented by S. Hell, Max Planck Institute Germany) However, I don´t get the idea here. Why could a segment of spherical wavefront be responsible for the longer-than-wide focal spot? By the way, my background is not in Physics so please make it as simple as possible. And sorry if there is a similar question already (I can´t find the search tab to look for it) Thanks a lot.