Question

Write about the main features in which the Fraunhofer and Fresnel approaches of diffraction differ.

Answer 1

Recall that diffraction is the process by which light seems to bend around the corners of an object and may impinge into the geometrical shadow of the object. Diffraction due to a straight edge and diffraction due to a narrow wire are examples of Fresnel diffraction, whereas, Young’s double slit experiment is an example of Fraunhofer diffraction.
Using these examples as a guide, deduce the characteristics of either processes in terms of distance to the obstacle, the type of incident wavefront and the change in the diffraction pattern for shift in object position for each of these cases.

Complete answer:
Let us begin by understanding what diffraction means in the first place.
Diffraction can be defined as the phenomenon of bending of light around the edges of an object leading to a subsequent encroachment of light into the geometrical shadow of the object. This happens when the light deviates from its straight-line path or when the geometrical shadow is as such not well defined.
Now, depending on the distance between the source of light and the screen from the obstacle which is a slit, there are two types of diffraction:
- Fresnel diffraction
- Fraunhofer diffraction

Let us look at their distinguishing characteristics:

| Fresnel Diffraction| Fraunhofer Diffraction
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1.| In this type of diffraction, the source of light and the screen are at effective finite distances from the diffracting object (i.e., the slit).| In this type of diffraction, the source of light and the screen are at an effective infinite distance from the diffracting object (i.e., the slit).
2.| For this reason, it is called near-field diffraction.| For this reason, it is called far-field diffraction.
3.| Since the light usually comes from a point source and the source is at a finite distance from the slit, the incident wavefront is usually spherical or cylindrical.| Since the source is at an infinite distance from the slit, the incident wavefront is usually a plane wavefront.
4.| The diffraction pattern is subject to change if there is any shift in the position of the object (i.e., the slit).| The diffraction pattern remains the same irrespective of any shift in the object (slit) position.
5.| The shape and intensity of the diffraction pattern change as the light gets propagated downstream to the source of scattering (i.e., the slit).| The shape and intensity of the diffracting pattern remains the same throughout.
6.| |

Note: Do not forget that though both Fresnel and Fraunhofer diffraction may seem widely different, they are united in the fact that, in both these processes, diffraction occurs in accordance with the Huygens-Fresnel Principle which states that every unobstructed point of a wavefront serves as a source of spherical secondary wavelets, and these secondary wavelets mutually interfere to produce the diffraction pattern seen on the screen. Thus spherical wavelets are formed in both cases and it's only the incident wavefront that is different for the two.