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difference between fresnel and frounhofer diffraction

what is Fresnel’s diffraction?

fresnel diffraction

fresnel diffraction of circular aperture

Fresnel diffraction can be observed if the source of light and the screen at which diffraction pattern is formed are kept at finite distance from the diffracting obstacle.In this situation the wave fronts falling on the obstacle are not plane.Similarly the wave fronts leaving the obstacle are not plane.

what is Fraunhofer diffraction?

Fraunhofer diffraction can be observed if the source of light and screen at which diffraction pattern is formed are placed at infinite distance from the diffracting obstacle.This can be done by using two converging lenses.One lens is placed between the source of light and the obstacle while the other lens is placed between the obstacle and screen.The lens between the source and obstacle makes the rays parallel to each other while the lens between the obstacle and screen ,focus the parallel rays at point on the screen.

Comparison table:

 

     Fresnel Diffraction   Fraunhofer diffraction
 1:If the source of light and screen are at finite distance from the obstacle ,then the diffraction called fresnel diffraction. 1:If the source of light and screen are at infinite distance from obstacle then the diffraction is called fraunhofer diffraction.
2:The corresponding rays are not parallel. 2:The corresponding rays are not parallel.
3:The wave fronts falling on the obstacle are not plane.                                                                                               3:The wavefronts falling on the obstacle are plane.

 

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