![]() In this case, the regions either closer or further from the lens give Fresnel diffraction. The Fraunhofer diffraction pattern is obtained at a very large distance from the aperture, but using a lens, an image of it can be formed at a finite distance. Calculation of Fresnel diffraction is based on an approximation, which eventually breaks down: closer to the aperture more advanced theories are required. For values of the Fresnel number well below 1, Fraunhofer diffraction occurs where the screen essentially shows the far-field. This situation is called Fraunhofer diffraction (shown in the image below).Ĭloser to the aperture the diffraction pattern does change with distance. If the observation screen is far enough away from the aperture, the diffraction pattern does not change in structure, but merely changes in size, as the distance is further increased. The light spreads as a result of diffraction. The amplitude at any point is the superposition of these wavelets.Ĭonsider an opaque screen illuminated with a plane wave. (i) Fresnel diffraction: In this class of diffraction, the source of light and the screen are at finite distance from the diffracting aperture or obstacle. No mirror or lenses are used for observations.ĭiffracted light is collected by a lens as in a telescope.Īccording to Huygens, each point on a wavefront serves as the source of a spherical secondary wavelet with the same frequency as the primary wave. Fresnel distance is the minimum distance which is travelled by a ray of light along linear path before diffraction. The wave-fronts are the plane which is realized by using the convex lens. The wave-fronts are divergent either spherical and cylindrical. The single-double plane diffraction grating is used. The source, and the screen is at an infinite distance from the diffracting aperture.įor obtaining Fresnel diffraction, zone plates are used. The source and the screen are at a finite distance from the diffracting aperture. The difference between Fresnel and Fraunhofer diffraction is as follows: Comparison and difference between Fresnel and Fraunhofer Diffraction S.No. Equating the above two equation gives, /aa/2z. From the definition of Fresnel’s distance, sin2 a/z 2 a/z. From the diffraction equation for first minimum, sin /a /a. The spreading out of light wave into the geometrical shadow when it passes through a narrow opening is known as the diffraction. Hence, Fresnel’s distance is the distance upto which ray optics is obeyed and beyond which ray optics is not obeyed but, wave optics becomes significant. Distinguish, differentiate, compare and explain what is the difference between Fresnel and Fraunhofer diffraction.
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