Editorial Feature

Diffraction - What it is and How it Works

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Diffraction is “the spreading and interference of light passing through gaps or bouncing off arrays of objects”, comparable in spacing distance or in size to the wavelength of the light. Diffraction is a characteristic phenomenon of waves (as opposed to the particle nature of matter). It is the bending of light around the edge of an object. The amount of bending is quite small but it depends on the size of the opening relative to the wavelength of the light. When the opening is large as compared to the wavelength of light, the amount of diffraction can be negligible. For small openings that are close in size to the wavelength of the light, the amount of diffraction is large. When the diffraction is large, it can be easily seen with our eyes. The optical effects caused by diffracted light commonly appear as dark, light or colored bands/regions. These bands are caused by interacting light waves, constructively or destructively during diffraction. When two wave crests meet or when two troughs meet, they combine to create a larger crest/trough. These are known as constructive interference. Combining a wave crest and a wave trough means they cancel each other out and there is no vertical displacement in either direction at that point. This is known as destructive interference. Typically, where constructive interference occurs the light is shown as a bright band or spot. Where destructive interference has occurred results in a dark band or spot.

Any wave gets diffracted around the bend. With advances in science, and discovery of other waves such as X-rays, gamma – rays, and a beam of electrons, diffraction progressed to the study of objects comparable to the size of the wavelength of the incident wave. For example, using X-rays, the ‘atomic’ arrangements of the material was revealed from the diffraction pattern. Thus, diffraction has become the tool of the century for advanced materials and atomic engineering.

The diffraction of light was first explained by the English scientist Thomas Young in the early 1800s. He made a pin-hole in his window shutter and observed diffraction of the sunlight on the board.

In 1912-1913, Sir William Henry Bragg and his son Sir W. Lawrence Bragg, at Leeds, achieved in giving rise to X-ray crystallography based on X-ray diffraction. They were awarded the 1917 Nobel Prize in Physics for this tool that brought about a revolution in studying the shapes and arrangements of molecules in crystals.

… at the 1913 meeting on “The Structure of Matter” William discussed his work with Albert Einstein and Marie Curie, along with several scientists, such as Leon Brillouin and Frederick Lindemann, who went on to make important contributions to the understanding of diffraction and crystal structure.

Philip Ball, The Birth of Crystallography

Max von Laue (from Germany) was awarded the 1914 Nobel Prize in Physics for “for his discovery of the diffraction of X-rays by crystals”. He showed that X-rays are wave-like in nature. With the characteristic diffraction pattern on photographic film, he showed that crystals have a lattice-like structure. Subsequently, the following year, another Nobel Prize in Physics in 1915 was awarded to William Henry Bragg and William Lawrence Bragg (father and son) for establishing the relation between the structure of the crystal and the wavelength of the X-rays, and the diffraction pattern. This relation has enormous practical implications. Bragg’s hypothesis predicted the diffraction pattern mathematically, based on the arrangement of atoms and the reflections from each successive plane of atoms within the crystal – it is known as the Bragg’s law. William Henry Bragg went onto design an X-ray spectrometer, collect reflections from several salts and proved the relationship between the X-ray diffraction pattern, and the atomic arrangement in a crystal that produced the pattern. This has led to the powerful tool of X-ray crystallography, celebrating more than a century of breakthroughs in science, as witnessed by the numerous Nobel Prizes involving it.

In the 1920s, scientists, Clinton Davisson and Lester Germer, from Bell Labs (New Jersey) observed diffraction of ‘quantum particles’, by firing an emission of electrons from a hot metal electrode on a regular array of objects spaced at a distance similar to the wavelength of the waves. The wave of electrons bounced off the array of elements, interfered with one another to produce light and dark regions. This Davisson-Germer experiment is a demonstration of the ‘wave-particle duality of electrons’.

The 1937 Nobel Prize in Physics was awarded jointly to Clinton Joseph Davisson and George Paget Thomson (the son of the late Sir J J Thomson, who discovered the electron!) for verifying independently the de Broglie’s bold thesis on electron diffraction, “by their experimental discovery of the diffraction of electrons by crystals”. In 1924, Louis de Broglie hypothesized the possible interpretation of the electron, a particle, as having wave nature as well; this wave-particle duality opened up new experimental possibilities.

Diffraction helps in atomic engineering, which enables understanding structure of many advanced materials that find applications in nanotechnology, electronics, and chemical catalysis. X-ray diffraction led to the structural determination of the deoxyribonucleic acids (in 1953) found in living organisms: its double helical structure was discovered. The structure of biological molecules means its function, which expands the understanding of the system, its physiology and also helps design possible drugs towards therapy.

Studying terrestrial and extra-terrestrial minerals, inorganic crystals, biological fibers, viral and mammalian proteins, has become routine characterization in Physics and Chemistry laboratories.

X-ray diffraction helps to inspect the piezoelectric ceramics that are used all around us in sensors and actuators.

Sources and Further Reading

This article was updated on 11th March, 2019.

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