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Diffraction | Types | Difference Between Fresnel and Fraunhofer

Introduction to diffraction

In general, the curvature of the waves around the edges of the barriers is known as the diffraction. The amount of flex depends on the wavelength of the waves and the size of the barrier. Bending is great in case of sound waves due to long waves. Light waves also bend to a smaller degree [due to shorter wavelengths], as they pass through sharp edges of obstacles or narrow openings. The curvature of light waves is more pronounced when the size of the aperture is equal to the wavelength of the light.

Diffraction at straight edge
Diffraction at straight edge


What are the different types of diffraction ?


According to Fresnel, the diffraction is due to the interference of the different secondary waves originating from the wavefront that are not obstructed by the obstacle. The diffraction phenomena are divided into two classes, they are (i) the Fresnel diffraction and (ii) the Fraunhofer diffraction.

different types of diffraction

(i) Fresnel diffraction - In such diffraction, the light source and screen diffraction are at a finite distance from the opening or barrier that has a sharp edge. The incident wave at the opening or barrier is spherical or cylindrical.

(ii) Fraunhofer diffraction: In such diffraction, the light source and the screen are infinitely far from the opening or barrier of the diffraction that has a sharp edge. This can be accomplished by placing the light source in the focal plane of the convex lens and display in the focal plane of the second convex lens. In this case, the wavefront event at the opening or barrier is a flat wavefront.

Comparison/Difference between Fresnel and Fraunhofer diffraction 

 

            Fresnel  Diffraction             

 

         Fraunhofer Diffraction       

 1.The point light source or an illuminated narrow slot is  used as the light source.

2.The light incident on the opening or barrier is spherical or cylindrical wave front .

3.Lenses are not used to focus the rays
1.Extended light source at infinite distance used as a light source

2.The incident light in the opening or obstacle is a plane wave front.

3.Converging lens is used to focus the rays


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