In the last section with interference, we (for the most part) only considered interference with a few waves. While there is no great physical distinction between interference and diffraction, it is customarily denoted a study of *diffraction* when you have interference of large numbers of wave. If this seems arbitrary, it’s because it is, but unfortunately I wasn’t around when Francesco Grimaldi first denoted the interference arising from a cone light propagating through two rectangular aperatures “diffractio”*. As you may note from the following contents of this section, this is going to be a wild ride, but I hope also one that makes the whole of classical optics make more sense**!

**Table of Contents:**

- Introduction to Diffraction
- Fresnel Approximation
- Paraxial Wave Equation
- Gouy Phase Shift
- Beam Power
- Gaussian Beam Propagation with ABCD Matrices
- Higher Order Gaussian Beams
- Integral Approach to Diffraction- Fresnel Diffraction
- Huygens’ Principle
- Fraunhofer Diffraction
- The Array Theorem
- Babinet’s Principle
- Diffraction in Paraxial Optical Systems
- Fourier Transform by a Lens
- The 4f Lens System
- Imaging of Extended Objects
- Near-field Imaging
- Fresnel Diffraction
- Fresnel Zones

*Actually, as much as we may dislike him for setting the tone of ambiguity about diffraction vs interference, Signore Grimaldi did some pretty cool experiments considering the year was 1665 when he published his work in “*De Lumine*“. For all of you fellow nerds out there, this article is somewhat of an interesting read into the actual experiments Grimaldi conducted. If you are really ambitious, I believe you can find Grimaldi’s original text in Latin somewhere in the void of the internet.

**Diffraction can again be described quantum mechanically as well; if you are interested in this picture, check out the section on Quantum Optics!