Diffraction of Waves

phenomena observed when waves pass by the edge of an obstacle and which are associated with a deviation of the waves from rectilinear propagation upon interaction with the obstacle. Because of diffraction, waves bend around obstacles, penetrating into the region of the geometric shadow. The possibility of hearing the voice of a person around the corner of a house is due to the diffraction of sound waves. The reception of radio signals in the long-wave and medium-wave bands far beyond the limits of direct visibility of the radiating antenna is due to the diffraction of radio waves around the surface of the earth.

Diffraction is a characteristic feature of the propagation of waves regardless of their nature. Diffraction can be explained in the first approximation by using the Huygens-Fresnel principle. According to this principle, in considering the propagation of a wave, every point of the medium that this wave has traversed may be considered a source of secondary waves.

Therefore, by placing a screen with a small aperture (having a diameter on the order of the wavelength) in the path of the waves, we will obtain in the aperture of the screen a source of secondary waves from which a spherical wave is propagated, also entering the region of the geometric shadow. If the screen has two small apertures or slits, the diffracting waves are superimposed on one another and, as a result of wave interference, produce a spatially alternating distribution of the amplitude maxima and minima of the resultant wave with smooth transitions from one to the other. As the number of slits increases, the maxima become narrower. When the number of equally spaced slits (the diffraction grating) is large, sharply separated directions of mutual wave amplification result.

Diffraction depends significantly on the ratio between the wavelength λ and the size of the object that causes diffraction. Diffraction is observed most distinctly in those cases when the size of the obstacles being rounded is commensurate with the wavelength. Therefore, the diffraction of sound, seismic, and radio waves for which this condition is almost always satisfied (X extends from approximately a meter to a kilometer) can be easily observed, while it is much more difficult to observe the diffraction of light (λ ~ 400-750 nanometers) without special devices. This same factor leads to many technical difficulties in studying the wave properties of other objects. For instance, insofar as X rays have a wavelength ranging from hundreds of angstroms to 0.00001 angstrom, it is impossible to manufacture a diffraction grating with such a distance between the slits, and therefore the German physicist M. von Laue used as a diffraction grating for studying X-ray diffraction a crystal in which the atoms (ions) were arranged in regular order.

Diffraction has played a major role in the study of the nature of microparticles. It has been experimentally established that when microparticles (such as electrons) pass through a medium (a gas or crystal), diffraction is observed. The diffraction of the particles is a consequence of the fact that microparticles have a dual nature (the so-called particle-wave dualism): in some phenomena the behavior of microparticles may be explained on the basis of a particle representation, while in others, such as diffraction phenomena, the wave representation may be used. According to quantum mechanics, to every particle there corresponds a so-called de Broglie wave, whose length depends on the energy of the particle. For example, a de Broglie wave with a length on the same order as the dimension of an atom corresponds to an electron with an energy of 1 electron volt. The diffraction of electrons and neutrons is used extensively to study the structure of matter.