Diffraction

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. In general, diffraction is defined as the spreading or bending of waves as they pass round the edge of an obstacle or through an opening whose size is roughly the same as the wavelength of the waves. The disturbed waves then interfere to produce ripple-like patterns. Diffraction is a property that distinguishes between wavelike and particle-like behavior.

Diffraction, in physics, is a phenomenon associated with the deviation of the propagation trajectory of waves when they encounter an obstacle in their path. It is typical of all kinds of waves, such as sound, waves on the surface of water or electromagnetic waves such as light or radio waves; the phenomenon also occurs in the particular situations in which matter shows wave-like properties, in accordance with the wave-particle dualism.

Diffraction effects are relevant when the wavelength is comparable with the size of the obstacle: in particular for visible light (wavelength around 0.5 ┬Ám) diffraction phenomena occur when it interacts with objects of sub-millimeter size.

Qualitative characteristics of diffraction

When faced with a diffraction phenomenon, in the optical case, some preliminary observations can be made. The general case of the phenomenon is Fresnel diffraction (or near-field diffraction), where the light source and the observation plane are placed at a finite distance from the slit. Fraunhofer diffraction (or far-field diffraction), however, is a special case of the previous one, but much easier to analyze: it occurs when the source and the plane are placed at infinite distance from the aperture, so that the incident rays can be considered parallel to each other. An example of this case is that of a point source of light (or rectilinear), such as the straight section of the filament of a light bulb or a laser beam, seen from a distance of a couple of meters through two blades half a tenth of a millimeter apart. The characteristics of diffraction are thus that:

  • the width of the central maximum of the single slit diffraction figure is twice the width of the side bangs.
  • the width is inversely proportional to the width of the slit: to very small slits correspond very wide diffraction bangs and vice versa.
  • the angles under which the bangs are seen, do not depend on the scale of the experiment, but only on the ratio between the wavelength and the width of the slit.
  • in any Fresnel phenomenon, a symmetric obstacle always presents light at the center of the shadow (it is the typical case of “Poisson spot”).

Acoustic diffraction

The diffraction of sound conceptually does not differ from the diffraction of light, but, since sound wave lengths are between 23 m and 2.3 cm, it affects objects and holes of much larger size than in the case of light. A typical phenomenon of sound wave diffraction is observed when a sound, arriving at a listener’s ear in a precisely lateral direction, also stimulates the other ear, albeit to a lesser extent: in this case the diffracting structure is the listener’s head. Using ultrasound, whose wavelength can become shorter than that of light, we can observe all the diffraction phenomena described for light waves.

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