## What is meant by Fresnel distance in optics

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What is meant by Fresnel distance in optics IE diffraction how is it related to the validity of ray optics please do make it simple as I am a beginner?

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## What is meant by Fresnel distance in optics

The Fresnel diffraction in optics is expressed by an equation for near field diffraction and it is an approximation of Kirchhoff-Fresnel diffraction which can be employed to the propagation of waves in the near field.

The near field is denoted by the Fresnel number, F of the optical arrangement that can be defined as for a wave incident on an aperture:

where

is the characteristic size of the aperture
is the distance of the observation point from the aperture
is the wavelength of the wave.

The several Fresnel diffraction causes the specular reflection at the nearly placed periodical ridges and this result  may be employed for atomic mirrors.