Question

How many lenses are there in Fresnel diffraction?

Answer 1

When light passes around an edge of an object it bends instead of going in the straight line. This phenomenon is called diffraction. The diffraction of light is classified into two types: Fresnel diffraction and Fraunhofer diffraction. The lenses can be used or not for these diffractions.

Complete step by step solution:
The Fresnel diffraction occurs when there is a finite distance between the slit and the light source. Also, the screen is at a finite distance from the slit or the obstacles. In this diffraction, the wavefront that incidents on the screen is circular or cylindrical in shape.
In Fresnel diffraction, there is no usage of lenses. The interference occurs between different light waves reaching a point from a similar wavefront. In this case, the phase of all the waves is different.

Hence, There is no lenses required in fresnel diffraction

Note: The differences between the Fraunhofer diffraction and the Fresnel diffraction are: (1) The distance between the slit or the obstacles and the sources or the screen are infinite for the Fraunhofer diffraction, whereas finite for the Fresnel diffraction. (2) A convex lens is used in Fraunhofer diffraction but no lens is used in Fresnel diffraction. (3) the wavefront is plain for the Fraunhofer diffraction but the wavefront is circular or cylindrical for Fresnel diffraction.
The diffraction pattern for the single slit Fresnel Diffraction is the same as the single slit Fraunhofer Diffraction, except the areas of minimum intensity are partially dark. This is due to there never being complete destructive interference in Fresnel Diffraction.