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What is the difference between Fraunhofer and Fresnel diffraction ?

This page compares Fraunhofer diffraction and Fresnel diffraction and mentions difference between Fraunhofer diffraction and Fresnel diffraction.

Introduction: Let us understand diffraction when wave encounters an obstacle. Bending of waves around the corners in this situation is referred as diffraction. Here object becomes secondary source of propagating wave.

With respect to light waves, the bending of light across edges of an obstacle is called "diffraction". In other words, it is the encroachment of light in the region of geometrical shadow of an obstacle.

Light wave Diffraction

The figure depicts diffraction of light waves. There are two major types of diffraction viz. Fraunhofer and Fresnel.

What is Fraunhofer diffraction

Fraunhofer diffraction

In this type, light source and screen are at infinite distance from the obstacle. It occurs with plane wave fronts with object effectively at infinity. The pattern is in a particular direction and it is a fringed image of the source.

What is Fresnel diffraction

Fresnel diffraction

in this type, light source and screen are at finite distance from the obstacle. It is produced when light from a point source meets an obstacle. The waves are spherical and pattern observed is a fringed image of the object.

Difference between Fraunhofer diffraction and Fresnel diffraction

Following table summarize difference between Fraunhofer diffraction and Fresnel diffraction.

Fraunhofer diffraction Fresnel diffraction
Source and screen are at infinite distances from slit. Source and screen are at finite distances from slit.
Incident wavefront on the aperture is plane. Incident wavefront on the aperture is either spherical or cylindrical.
The diffracted wavefront is plane. The diffracted wavefront is either spherical or cylindrical.
Two biconvex lenses are needed to study diffraction in lab. No lenses are needed to study diffraction in the lab.
Mathematical treatment is easy. Mathematical treatment is complicated.
It has many applications in designing the optical instruments. It has less applications in designing the optical instruments.
The maximas and minimas are well defined. The maximas and minimas are not well defined.


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