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(This page is under development, and might be completed Winter 2002).
Background
Diffraction is the term used to describe the spreading of light at the edges of an obstacle. First described in detail in the 1665 posthumous book by the Italian professor of mathematics, F. Grimaldi, diffraction was ultimately to supply the incontrovertible proof for the wave nature of light. Grimaldi studied the shadow of small objects using a small opening in a window shutter (just as did Newton to obtain the spectrum a few years later). He observed that these shadows were larger than could be accounted for by geometrical considerations and noted colored fringes, not only outside but even inside the shadow under certain conditions; he coined the word "diffraction" for the effects that he was not able to explain. A series of investigations followed, but it remained for the three contemporaries Thomas Young (1773-1829), Joseph von Fraunhofer (1787-1826), and, particularly, Augustin Fresnel (1788-1827) to provide adequate descriptions and explanations.
Consider sunlight passing through an aperture and falling onto a screen. As the opening is made smaller, so does the patch of light on the screen become smaller; at the same time its edges appear to become sharper. Beyond a certain point, however, the edges become indistinct and begin to show colored fringes. The mechanism involves the spreading of a wave into the geometrical shadow region of an object, as shown in Fig. 36. A single edge in monochromatic light produces a sequence of light and dark bands, while in white light a sequence of colors is produced much as in the Newton color sequence discussed above.
When light beams diffracted from opposite sides of small particles interfere, the result is a sequence of colored rings called the "corona" (not to be confused with the "solar corona" seen only during an eclipse), usually seen around a bright light source. This can frequently be seen surrounding the sun (using very dark sunglasses), derived from cloud particles. In Fig. 37 the path difference between the two diffracted rays at A is zero, thus producing reinforcement. At B the path difference is lambda = d sin theta; when this is a whole wavelength, the second reinforcement occurs, while in between there will be cancellation. The size of the cloud particles can be determined from observing the diameter of the corona. With a range of sizes of particles present, merely a bluish disk, the "corona aureole," is seen surrounding the sun. The same mechanism explains Bishop's ring, the glory, and the specter of the Brocken. Transluscent clouds passing near the sun having uniform particle size may show a range of pastel colors, then called iridescent clouds (very dark sunglasses).
A diffraction grating consists of a regular two- or threedimensional array of scattering objects or openings. This is most familiar when used in a spectroscope as in Fig. 38, where the path difference is n lambda = d sin theta. Diffraction arrays are also observed in the animal kingdom, as in the beetle Serica sericae and in the indigo or gopher snake Drymarchon corais couperii, which show spectral colors in direct sunlight. The same effect can be obtained by a glancing view across a phonograph record or on viewing a distant streetlamp or flashlight through a black-cloth umbrella, as in Plate XV.
Opal
The most outstanding diffraction grating of all is the gemstone opal, showing on a white or a black background flashes of varied colors called the "play of color," as seen in Plate XVI. This was at one time thought to involve thin-film interference, but electron-microscope photographs taken of an opal reveal its secret, demonstrating a regular threedimensional array of equal-size spheres as shown in Fig. 39. The actual composition of the spheres is amorphous silica, SiO2, containing a small amount of water; the spheres are cemented together with more amorphous silica containing a different amount of water so that a small refractiveindex difference exists between the spheres and the cement.
Liquid crystals
Finally, there are "liquid crystals," organic compounds with a structure intermediate between that of a crystal and that of a liquid. These have twisted structures, which can intereact with light in a manner similar to diffraction to produce color, as in the thermometric film of Plate XVII, -and in "mood" rings. Some beetle colors are derived from liquid crystal structures on the outer layers of their cuticles.
FIG. 36. The geometrical and wave shadows of an opaque object produced by particles and waves, respectively.
FIG. 37. Diffraction by drops of water leading to the corona.
FIG. 38. A diffraction grating spectroscope showing constructive reinforcement for n = 0, the zero-order line, and for n = 1, part of the first-order spectrum.
PLATE XV. Diffraction of the light from a distant flashlight viewed through a black umbrella fabric.
PLATE XVI. Colors produced by diffraction in a 12-mmdiameter synthetic black opal, grown at Ets. Ceramiques Pierre Gilson.
FIG. 39. Electron-microscope view of a synthetic opal; individual spheres are about 250 nm across. [Photograph courtesy Ets. Ceramiques Pierre Gilson.]
PLATE XVII. Skin temperature revealed by thermography of a hand pressed against a black-backed cholesteric-liquidcrystal film; the iridescent metalliclike colors are derived from multiple-layer diffraction.
