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Radiology => Electromagnetic Radiation => Diffraction
Diffraction
Diffraction, property of wave motion, in which waves spread and bend as they pass through small openings or around barriers. Diffraction is more pronounced when the opening, or aperture, or the barrier is similar in size to or smaller than the wavelength of the incoming wave. Diffraction is a property of the motion of all waves. For example, if a radio is turned on in one room, the sound from the radio can be heard in an adjacent room even from around a doorway. Similarly, whenever water waves pass an object on the surface of the water, such as a jetty or boat dock, waves that pass the object's edge spread out into the region behind the object and directly blocked by it.
To understand this effect, Dutch physicist Christiaan Huygens proposed that each point of a wave on a flat wave front, or crest, acts like a source of secondary, spherical wavelets, or smaller waves. Before reaching a barrier, these secondary wavelets add to the original wave front. When the wave front approaches an aperture or barrier, only the wavelets approaching the unobstructed region can get past the barrier. When the size of the opening or barrier is large compared with the wavelength of the incoming wave, the sum of the wavelets passing through the aperture is nearly flat. The resulting wave front resembles the original wave front, and little bending occurs. However, when the size of the opening is comparable to or smaller than the wavelength of the incoming wave, it appears as though only a few wavelets can get through. These remaining wavelets are then a source of more wavelets that expand in all directions, and the shape of the new wave front is curved. The wavelets of these diffracted, or bent, waves can now travel different paths and subsequently interfere with each other, producing interference patterns. The shape of these patterns depends on the wavelength and the size of the aperture or barrier. According to Huygens's principle, diffraction can be thought of as the interference of a large number of coherent wave sources. Consequently, diffraction and interference are essentially the same phenomenon.
Diffraction of waves is useful in crystallography and solid-state physics. The regular ordering of atoms in crystalline solids (such as NaCl, or common table salt) acts like a geometric array of small barriers that diffract light. X rays are electromagnetic waves the wavelengths of which are comparable in size to the spacing between the atoms in crystals. When these waves pass through crystals, they diffract and form interference patterns. These patterns can then be analyzed to gain information about the geometric structure and properties of the crystals. Much of what is known about the structure of solids is due to this application of X-ray diffraction. X-ray diffraction has led to the discovery of the double-helical structure of DNA and is also used in holography.
Diffraction can limit the resolving power of microscopes and telescopes. If the observed object or the aperture of the observing device is comparable in size to the wavelength of the reflected light, the light will diffract rather than produce a clear image. This problem can be minimized by making the aperture of the device as large as possible or by reflecting light of smaller wavelengths off the observed object. For even higher resolution and magnification, diffraction effects can be reduced by using the wave properties of electrons. Because the effective wavelength of electrons can be 100,000 times shorter than the wavelength of visible light, electron microscopes can produce much sharper images than are possible with visible-light microscopes.
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