In optics, the Fresnel diffraction equation for near-field diffraction, is an approximation of Kirchhoff-Fresnel diffraction that can be applied to the propagation of waves in the near field.
The near field can be specified by the Fresnel number, F of the optical arrangement, which is defined, for a wave incident on an aperture, as:
where
- is the characteristic size of the aperture
- is the distance of the observation point from the aperture
- is the wavelength of the wave.
When the diffracted wave is considered to be in the near field, and the Fresnel diffraction equation can be used to calculate its form.
The multiple Fresnel diffraction at nearly placed periodical ridges (ridged mirror) causes the specular reflection; this effect can be used for atomic mirrors.
Read more about Fresnel Diffraction: Early Treatments of This Phenomenon, The Fresnel Diffraction Integral