Diffraction | Types of Diffraction


What is Diffraction? What are the Difference between diffraction and interference? What are the types of Diffraction?

Answer:

Diffraction

In the year 1660 Italian scientist Francesco Maria Grimaldi observed diffraction pattern. In the classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Similarly the diffraction phenomenon is described as the interference of waves according to the Huygens–Fresnel principle. Diffraction effect depends upon the size of obstacle. These characteristic behaviors are exhibited when a wave encounters an obstacle or a slit that is comparable in size to its wavelength. Light waves are very small in wavelength, i.e. from 4 x 10-7 m to 7 x 10-7 m. If the size of opening or obstacle is near to this limit, only then we can observe the phenomenon of diffraction. So the diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, X-rays and radio waves.

Diffraction

Difference between diffraction and interference

  • Diffraction occurs when a wave meets an obstacle or a slit, whereas Interference is the phenomenon where waves meet each other and form constructive and destructive interference.
  • In interference the region of minimum intensity is perfectly dark whereas in diffraction they are not perfectly dark.
  • Width of the fringes is equal in interference, whereas In diffraction they are never equal.
  • Diffraction happens around edges or holes, interference happen at the intersection of two waves.
  • Two separate wave fronts originating from two coherent sources produce interference; whereas Secondary wavelets originating from different parts of the same wave front constitute diffraction.
  • The intensity of all positions of maxima is of the same intensity in interference, whereas in diffraction they do vary.

Diffraction is a phenomena arising from the wave nature of light. A diffraction pattern appears because light will interfere with itself, just as waves do.

There are two types of diffraction: Fraunhofer diffraction and Fresnel diffraction.

  • Fraunhofer diffraction:

If the source of light and the place where you’re receiving the light must be relatively far from the obstruction (e.g. >1 meter from a 0.1 millimeter slit or hole), this pattern is a Fraunhofer diffraction pattern and it is also known as “far-field” diffraction. These distances must be large enough so that the lights that arrives and leaves the obstruction and reaches the wall or screen are nearly plane waves.

Fraunhofer diffraction is also characterized by a linear variation of the phases of the Huygens secondary waves with distance across the wavefront, as they arrive at a given point on the observing screen. For example the light can occur as plane waves, which we can imagine as the waves that come rolling in over the ocean. These plane waves can hit an obstruction, like when ocean waves hit a dock, and travel on in a very different pattern. At a large (compared to the size of the obstruction) distance away from the obstruction, there will be an illumination pattern of light and dark depending on the direction from the obstruction. This pattern is a Fraunhofer diffraction pattern.

  • Fresnel diffraction:

If the distance between the obstruction and the screen can be arbitrarily close, but the interesting distances are on the order of centimeters to 1 meter from a millimeter sized obstruction. Because these distances are not too far, this pattern is a Fresnel diffraction pattern and it is also known as “near-field” diffraction.

Fresnel diffraction requires for its observation only a point source, a diffracting screen of some sort, and an observing screen. The latter is often advantageously replaced by a magnifier or a low-power microscope. The observed diffraction patterns generally differ according to the radius of curvature of the wave and the distance of the point of observation behind the screen. So the diffraction effects obtained when the source of light or the observing screen are at a finite distance from the diffracting aperture or obstacle come under the classification of Fresnel diffraction.

 

 Difference between Fraunhofer diffraction, and Fresnel diffraction,

Fraunhofer diffraction

  • Source and the screen are far away from each other.
  • Incident wave fronts on the diffracting obstacle are plane.
  • Diffracting obstacle give rise to wave fronts which are also plane.
  • Plane diffracting wave fronts are converged by means of a convex lens to produce diffraction pattern.

Fresnel Diffraction

  • Source and screen are not far away from each other.
  • Incident wave fronts are spherical.
  • Wave fronts leaving the obstacles are also spherical.

Convex lens is not needed to converge the spherical wave fronts.

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