Editing Huygens–Fresnel principle (section)

==Huygens' principle as a microscopic model==
The Huygens–Fresnel principle provides a reasonable basis for understanding and predicting the classical wave propagation of light. However, there are limitations to the principle, namely the same approximations done for deriving the [[Kirchhoff's diffraction formula]] and the approximations of [[Near and far field|near field]] due to Fresnel. These can be summarized in the fact that the wavelength of light is much smaller than the dimensions of any optical components encountered.<ref name="Born and Wolf"/>

[[Kirchhoff's diffraction formula]] provides a rigorous mathematical foundation for diffraction, based on the wave equation. The arbitrary assumptions made by Fresnel to arrive at the Huygens–Fresnel equation emerge automatically from the mathematics in this derivation.<ref>{{cite book |first1=M. V. |last1=Klein |first2=T. E. |last2=Furtak |title=Optics |year=1986 |publisher=John Wiley & Sons |location=New York |edition=2nd |isbn=0-471-84311-3 }}</ref>

A simple example of the operation of the principle can be seen when an open doorway connects two rooms and a sound is produced in a remote corner of one of them. A person in the other room will hear the sound as if it originated at the doorway. As far as the second room is concerned, the vibrating air in the doorway is the source of the sound.