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Thomas B. Brill University of Delaware Newark, DE 19711
Why Objects Appear as They Do
When one examines the surface of an ohject, attention is usually focused on color, texture, degree of transparency, and other similar characteristics. Such a descrintion is reallv based on how the eye perceives light to interact with the suriace. An indivisible connection is created between lieht and the ohiect. In art, crafts, and museum work where display is intended, creat effort is made to hirhlirht a work and nttrart admirers through visual stimulus. fineobtains these effects, knowingly or unknowinelv, hv manipulatine- lieht - at the surface and in the body of a n b j e c t . The coal of this article is to show how some of the optical effeds in art, antiquities, and nature can he understoodusing chemical and physical concepts frequently encountered in the classroom or laboratory. Chemistry and physics become inseparable as do the past and present. I t is regrettahly impossible to nresent a detailed nicture of this vast suhiect in a short article, hut some of the most important influences on color, contrast, and degree of transparency can be accented. ~
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The Origins of Color
Color is an integral part of most objects. Nature, artists, and craftsmen have created it in manv. wavs. . Piements and dves rome LO mind as the dominant sources of color. They act to selectively ohsorh specific wavelengths of visible light while reflecting the remainder. However, color is also produced by materials which are incapable of ahsorbing visil~lelight. In these instances interference patterns, Rayleigh scattering, and other phenomena which rrdirvcr rather than absorb wlectrd wavel&ths are responsible. Let us consider color by the phenomena that produce it. Visible light is absorlred by compounds when electrons are excited from the around state toexcited states. The absorhed energy usually i;converted to heat. The magnitude of the energy gap hetween the states determines the energy (color) of light removed from the spectrum. However, the color ahsorbed is not the color observed. Instead we observe the complement of that color. Complementary colors are easily identified in the color rosette (Fig. 1) which we owe primarily to Isaac Newton. The colors are arranged in a circle with a clockwise increase in energy from red to violet. Any two diametric colors are said to he complementary. ( Red and green are, for example, complementary colors). An idealized mixtue from which, say, red and green are removed, produces white light. A useful application of the color rosette lies in understanding the production of color by absorption. The color absorhed hva material is the complement of the color we ohserve. If an ohject absorbs red, we observe it to he green. Similarly if green is seen in the light reflected from a semi transparent ohject, then red is seen in the light transmitted through it. The Lycurgus Cup helonging to the British Museum in London illustrates this nhenomenon (see front cover for color photo). The object depicts the mythological Spartan lawgiver, Lycurgus, entwisted in a vine. I t is a Roman glasswork fahricated in the 4th or 5th century A.D. Light passing throueh thin transnarent sections of the elass is red, while greenis seen in theiight reflected from thicker, more opaque sections.
Figure 1. The color rosette shows the colors corresponding to a clockwise increase in energy from red to violet. Interference
Another source of color occurs when light waves interfere with one another. Interference patterns produce the color of soap bubbles. oil films on water.. ovster shells. certain beetles " and butterflies, camera lenses, surfaces of polished metals which haw been heated in air, and. t o some~extmt,peacock ftmhers and mother-of-pearl.Anrient glass and rhe Lusterwnrrs of Tiffmy also shwv beautiful iridescence hecausr of intrrtrence parterns. In all of thrse exarnples,abaorpt~mof lieht is not rcir)maihle for the color. Instead red~rcrtionof seiected wavelengths occurs. Interference takes place when a thin film of a transparent substance exists iilune or un a reflectin:: surface. Light wnws rrflerted from rhe u v w r surface of the film mav he either in phase or out of phase kith the waves reflected from the lower surface. Whether constructive or destructive interference occurs depends on both the thickness of the film and the wavelength of light. In practice, a wide range of spectral colors is usually seen because the film varies in thickness. Any wavelength of light which destructively interferes with itself in the reflection Drocess will he absent from the reflected light. Hence we obser;e its complementary color. The wavelengths which destructivelv interfere during reflection are not lost: instead they appear in the transmitted light. Therefore, just as occurred in the absorvtion nrocess discussed above. the colors in reflected and transmiited light are complementary. Interference acts to redirect lieht into two comnlementarv components, one being reflecteh and the other being transA...
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Louis Comfort Tiffany became famous for his production of Lustenvare in the U S in the 18851928 period. His method was to apply a thin coat of metal oxide to a glass or ceramic surface, and then heat the work in areducing atmosphere. The metal oxide converted to a verv thin film of metal which was capable of producing interference patterns. A strikingly opalescent surface resulted. Ancient glass which has been buried in damp soil is frequently beautifully iridescent. The surface became leached by moisture, leaving extremely thin sheets of glass in place of the original solid piece. These thin sheets create interference Volume 57, Number 4, April 1980 / 259
Figure 2. The relative sensnivity of me "typical" eye to equal intensities of bright visible llght.
patterns in the reflected and transmitted light. Polished metals which have been heated in air develop a thin sheet of metal oxide with varying thicknesses depending on the temperature. This layer of metal oxide creates interference patterns, the color of which can aid the metallurgist in determining the temperature to which the object was heated. It sometimes adds a decorative element to the object, as well. Rayleigh Scattering When pnrticlesare dispersrd ina gas, liqu~d,or solid, light is affected in different w a y depending on the size of the oart~clri.If the narriclesaresmallerthan0.1 A. uzhereA is the wavelength or the incident light, then the light is subject to scattering acmrdine to Rdvleirh's laws. In 1871. Lord Ravleiah theorize; that the Yntensky G t h e light scattered at right &gles from small particles varies
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1) directly with the intensityof the incident light 2) directly with the square of the average volume of the parti-
cles 3) inversely with the fourthpower of the wavelength of the incident
light (Ilk4). The most broadlv aonlicahle result of Ravleieh's work is contained in (3), whiiisays that short waveiengths are more effectivelv scattered than lone waveleneths. For example, violet light with a wavelength of 4% nm (nm = nanometers) is scattered about 10 times more eificientlv than red lirht - at 625 nm. Rayleigh's theory helped explain many phenomena. For instance, the colors of the sky had attracted the attention of natural philosophers for centuries. Rayleigh's work settled the question. He concluded that the sky is not inherently blue. Instead, water droplets and dust particles are present in the atmosphere in the 0.1 A size range. A is in the visible region of 400-700 nm. Sunlight containing all wavelengths encounters these particles. The shorter waveleneths - .(blue and violet) are scatt&ed toward the observer more efficiently than are the loneer waveleneths (red and oranae) when viewing the sky away from the sun. When viewingthe sunset or sunrise, the eve is seeing the lieht that passes throuah the particulate matter. since the hl"e light ismostly scatteied to
