What is Diffraction?

Diffraction is the deflection and subsequent spreading of a beam of radiation when the beam grazes the edge of an obstacle or passes dirough a narrow aperture. Diffraction applies to radiation in die form of light waves, sound waves, and all other kinds of waves. Scientists make use of diffraction effects in spectroscopic work and in studies of the atomic structure of solids.

History

The first observation of diffraction is attributed to Francesco Grimaldi (1618-1663), who discovered it by observing sunlight passed through a pinhole aperture. An explanation of diffraction was furnished by Christian Huygens (1629-1695); according to Huygens' principle, the progress of a light wave can be predicted by assuming that each point in a wave front acts as the source of a secondary wavelet that spreads out in all directions. The envelope of all the secondary waves produced by all of the points is the new wave front. This principle not only explains diffraction but also the basic laws of refraction and reflection.

Diffraction of Light

Diffraction is easily demonstrated by using a point source of light, an object, and a white screen. When the object is placed between the light source and the screen, the shadow cast by the object is not perfectly sharp because some of the light spreads into the geometric shadow region (see figure on the following page).

When an opaque partition containing a small pinhole or slit is used, the light from a point source spreads out after passing through the slit. In this case, the finer the slit, the greater is the angle of spread.

When the aperture consists of a number of regularly spaced openings, as in a fine mesh screen, a combination of diffraction and interference occurs. The light passing through the screen is broken up into a definite pattern, and each part of the pattern is dispersed into a spectrum of colors. Optical diffraction gratings for spectroscopy make use of this effect. Such gratings are made by ruling many fine lines on a glass plate, typically 50,000 or more over a width of 2 or 3 inches (5 or 7.6 cm). The parallelism and spacing of the rulings must be accurate within a few millionths of an inch to be useful for precise spectroscopic work.

Diffraction of X-Rays

In a crystalline solid, the atoms occur in a regular periodic arrangement called a lattice. The lattice acts much the same as a grating when radiation passes through the crystal or is reflected from its surface; however, the crystal structure must be studied with very short wavelength radiation, namely, X-rays, because of the extremely small spacing between adjacent rows of atoms. The precise arrangement of atoms in a crystal is now known as a result of X-ray diffraction studies.

Diffraction of Electrons

Diffraction effects are also observed when a beam of electrons is sent through matter. Electron diffraction, like X-ray diffraction, is useful in the study of the atomic makeup of solids.

Diffraction of Light

When a light beam from a point source grazes the edge of the opaque obstacle, some light rays are deflected and spread downward, illuminating a part of the region in back of the opaque obstacle.

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