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Diffraction

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If we observe the shadow generated by an opaque object has alternate bright and dark bands are present similar to the pattern of interference. This is due to the diffraction. All types wave such as a sound wave, light wave and matter wave etc exhibit the diffraction phenomena. Since, the light wavelength is smaller than that of the obstacle size, we do not encounter diffraction effects of light in everyday observation. 

Diffraction Definition

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Diffraction is the bending of a wave as it moves around an obstacle or passes through a narrow opening.
Diffraction Sound Radio Waves

A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle. If the wavelength is small compared to the opening or obstacle, the wave bends very little. The larger the wavelength is compared to the size to the opening or obstacle, the more the wave diffracts.

Fraunhofer Diffraction

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The diffraction phenomena are divided into two classes:
  • Fraunhofer diffraction
  • Fresnel diffraction.
The diffraction in which the source of light and the screen are effective at infinite distances from the obstacle or aperture, causing the diffraction, is called Fraunhofer diffraction.

As the source is at infinity the waves reaching the obstacle are plane waves. The curvature of wavefront is negligible.

Fraunhofer Diffraction

Fresnel Diffraction

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The diffraction in which either the source of light or the screen or both are at finite distances from obstacle or aperture, causing the diffraction, is called Fresnel diffraction. The source is at a finite distance, therefore the incident wavefront is either spherical or cylindrical. Hence, secondary waves are not in phase. The calculation of intensity of the resultant pattern on the screen is complex.

Single Slit Diffraction

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In Fraunhofer diffraction, a plane wavefront is considered. When parallel light is incident on a narrow slit, some light is observed in geometrical shadow due to diffraction. A converging lens is used to focus the rays on the screen and the intensity pattern is observed. The central region is of maximum intensity and it is followed by dark and bright bands of decreasing intensity. The pattern is symmetric about the axis.

Diffraction Light Single Slit

Double Slit Diffraction

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Double slit diffraction will occur when two slits are there to pass the light source. Here, the coherent source is incident on the slits, it passes the light rays and the intensity pattern is observed on the screen. The maximum intensity is at the center. The intensity decreases as moving towards the end.

Circular Aperture Diffraction

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Circular aperture diffraction takes place if the slit is in circular shape. In this case there is no dark and bright bands, but the intensity patterns are in the form of concentric circles with high intensity at the center. The circular intensity ring is termed as Airy's ring or disc. The given figure illustrates the circular aperture diffraction.

Circular Aperture Diffraction

Diffraction Grating

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An array of either apertures or obstacles capable of producing diffraction pattern is called diffraction grating. David Rittenhouse invented it in 1785. Later on Fraunhofer independently rediscovered it and made important contribution to the theory and technology of gratings. A system is equally spaced slits acts as a transmission grating. A transmission grating is manufactured by ruling a series of closely separated parallel lines on a glass plate.

Example of Diffraction

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Some examples related to diffraction are,
  • If we observe the CD at a particular angle, seven colours in rainbow is observed.
  • The contrast sound of lightning strike
  • Sound due to marching bands