In'ter-fer'ence, n. (From INTFRFFRE.) Physics. The mutual effect, on meeting, of two beams of light or of two series of pulsations of sound or, generally, of two waves or vibrations of any kind. When two beams of monochromatic light from the same source meet, especially if their waves have equal amplitudes, the neutralization of waves at some points and their reinforcement at others produces alternate light and dark lines, bands, or fringes. If the beams are composite, as in white light, the neutralization of certain wavelengths causes the lines, bands, or fringes to show colors. Cf. IRIDF-SCFNCE, 1. This is satisfactorily explained by the undulatory theory of light. In the case of sounds, interference produces silence, increased intensity, or beats. Webster's New International Dictionary Unabridged Edition. Through our increased knowledge of light in general we have also gained additional insights into interference since this phenomenon was discovered three hundred years ago. The credit for formulating the principle of iridescence, or the "colors of thin films" as he called them, goes to Robert Boyle, contemporary and compatriot of Sir Isaac Newton. More than a century later, interference was explained in detail by the English physicist Thomas Young. According to Young's definition, very thin plates or films, such as a coat of oil on water, or the "skin" of a soap bubble, reflect some of the incoming light from their mirror like top surface. The rest of the light travels through the film and is then reflected by the lower surface. The light that enters the film is bent and deflected from its path by the film's greater density, or refractive index The refractive index of air, which in physics is considered the norm, has been assigned the numeral 1, and anything, denser than air therefore has a higher numeral. Assuming the oil of an oil slick were twice as dense as air, it would then have a refractive index of 2; light traveling through the oil is bent, and slowed down, twice as much as the same light traveling through air. What actually happens when light enters a medium with a greater refractive index? The movement of the light is slowed down, the waves become smaller, and the traveling distance within the film is increased. In other words, the light cannot go straight through. After the light reaches the lower surface and is reflected, it has to make the return trip through the film, still at the same slowed-down pace. When this light finally rejoins the light that is reflected at the upper surface, it is out of phase. How much out of phase it is depends on the distance it has had to travel within the film, which in turn depends on two factors: the thickness of the film, and the angle at which the light strikes the surface. If the phase difference between the beams reflected at the upper and lower surf aces happens to equal one full wavelength, or a multiple thereof, that particular wavelength or color, in other words-will be reinforced when the light that went through the film rejoins the upper surface reflected light. Reinforcement will be strongest and the color purest if the waves happen to have the same amplitudes, that is, if their crests and troughs are of equal height. All the other wavelengths are either weakened considerably or, where crest meets trough, eliminated. This, of course, results in only one color becoming visible. For instance, assume that a green ray, after having traveled through the film and back, has a phase difference of two full wavelengths when it rejoins the green light reflected at the upper surface. It is now in phase with this green light: wave crests and wave troughs of both lower- and upper-surface reflected green light coincide. The green color is thus reinforced. Because the wavelengths of the other colors are different, they are by necessity out of phase and are therefore either eliminated or very much weakened. However, if the phase difference between the two green light rays were one and a half wavelengths, they would neutralize each other and become invisible. In that case, some other color whose wavelengths coincide would appear. These facts hold true only for composite light. If the light is monochromatic-if it consists of only one color-the result of interference would be the dark lines, bands, and fringes mentioned in the dictionary definition. Darkness results when waves of the same amplitude are out of phase in such a way that wave crest meets wave trough. We thus have the apparently paradoxical situation that light plus light may equal darkness. We have seen that the color of light reflected by a thin film can be changed simply by increasing or decreasing the distance the light has to travel within the film. Every change of this "traveling distance" causes a different wavelength to be in phase. This can be achieved in a number of different ways. One of them is to alter the thickness of the film. This, however, has its limitations, because films that are too thin reflect all the light, and those that are too thick eliminate only a few colors, with the result that the remaining mixture again appears colorless. Shifting the angle of light incidence is another way of changing the traveling distance, and with it the phase difference, of that part of the light that has to go through the film and back to the upper surface. With every change of the optical distance a different color appears, and this is the phenomenon known as iridescence. Common occurrences of iridescence in our daily lives are soap bubbles, with their rapidly changing gleaming, pure colors, and the familiar oil slick on a wet street. Both are classic examples of the "colors of thin films." Diagrams can provide a clearer picture of what happens when light beams interfere. Fig. I shows two beams of light striking the surface of a film, whose density, and therefore whose refractive index, is greater than that of air. While beam No. I is reflected at the upper surface, beam No. 2 enters the film, is refracted, and is slowed down by the denser medium. Traveling to the lower surface, beam No. 2 Is reflected there and returns to the upper surface, where it rejoins beam No. I at the point of greatest optical density, here marked with the letter C. The distance ABC minus CD is the phase difference between the two beams. Let us assume that this distance equals one full wavelength of green light. This would mean that the green light waves of beam No. I are in phase with the green light waves of beam No. 2 when they meet at point C. Because their wave crests and troughs coincide, what we now see is a pure green color, especially if the waves have the same amplitudes. All other colors, being out of phase, have been more or less eliminated. In Figs. 2 and 3, the angle of incident light has been changed. This changes the traveling distance for beam No. 2 within the film in each case. It follows that the phase difference is also changed, and that, in turn, means that different colors appear. Fig. 4 shows that the optical distance can also be changed by substituting a thinner film. In this case, it is shortened, with the result that still an- other color appears even if the angle of light remains constant. This explains why different colors appear and disappear in rapid succession as air is pumped into a soap bubble and the "skin" becomes thinner. All iridescence, regardless of whether it is found in organic or In inorganic matter, is based on the principle of interference, which causes light waves of composite light to reinforce, weaken, or eliminate each other alternately, depending on the phase differences and the amplitudes of the light waves involved. Thin films, however, are only one of a number of ways in which light may be brought to interfere and produce colors; physicists have found certain structures that affect light in a way that causes interference. When light strikes very thin slits, it is diffracted, or deflected from its path. Fig. 5 shows what happens in that case. Depending on the phase difference 4B, one color is reinforced and the others are weakened. If such slits are arranged at equal distances from each other, we get what physicists call a diffraction grating. The German physicist Joseph von Fraunhofer produced artificial diffraction gratings for measuring the component colors of white light. Fine parallel lines were scratched into a sheet of glass with the help of a diamond, and then a beam of white light was directed onto this glass at angles ranging from 0 to 90 degrees. As interference colors appeared, the wavelength of each color was determined by mathematical calculations that took into consideration the phase difference and the amount of diffraction. We now have examples of interference colors produced by thin films and thin slits. There is at least one additional way to get the same results, and this possibility for a long time nothing more than an optical theory is especially important to us here, because some of the most beautiful iridescent animal colors are based on this particular structure. Research on how the blue color of the sky is produced had already established that very minute particles, suspended in a medium with a different refractive index, cause light refraction. If these particles are suspended diffusely throughout the other medium, they can produce color, as in the sky and in blue feathers. They cannot, however, produce interference. The situation changes if such particles are arranged on levels, or planes, which are spaced at equal distances from each other. Now interference becomes possible in certain circumstances. A structure called a space lattice in optics has all the particles distributed in a cubic arrangement on planes that are stacked one on top of the other like layers in a layer cake. Interference by means of a space lattice is caused by arranging several of these planes in a way that will produce the right phase difference for whatever color is to appear. Changes in the angles of incident light as well as variations in the distance between layers can cause different colors to appear. The normal color is considered to be that which is produced when light enters the film at an approximately vertical angle. When light strikes a space lattice, one portion is reflected immediately from the top. Another portion is refracted into the underlying layers of the space lattice, where it is again refracted and reflected. A third portion enters directly between the particles into the lower regions of the space lattice and is reflected there. The more levels are stacked one on top of the other, the purer and more monochromatic the light reflected by the space lattice becomes, for physicists have proved that even neighboring wavelengths eliminate each other through interference. Sir William Bragg, the English physicist mentioned earlier, did much work on the problems of the space lattice. He found that the most important factor for production of interference was the maintenance of equal distances between levels. The position of the particles in each level was relatively unimportant. Because a space lattice calls for submicroscopic precision structures, it long remained simply a theory, although physicists did find evidence of a natural space lattice while examining crystals with ultraviolet rays. No present-day machine can produce a structure that would come close to fulfilling the conditions necessary for creating a space lattice. With the development of the electron microscope came the possibility of examining submicroscopic structures in organic as well as in inorganic matter. It did not take long for scientists to find that, while creation of a space lattice may be beyond the capability of man even today despite all the advances in science, these, as well as other structures that produce interference colors, are a common occurrence in living organisms. In fact, they are being produced by the billion daily in various animal tissues ranging from scales to feathers! Interference colors are the purest and most brilliant colors known to man. They cannot be matched by even the brightest pigment colors in depth and intensity. In addition, the glittering play and change of hue that accompanies any change of light angle or observer position lends these colors a magic and beauty unparalleled by any others. It is, therefore, quite understandable that animals displaying such colors were objects of admiration and wonder long before people knew and realized that the infinitesimal structures necessary to create these colors-unmatched by anything human beings can create-are, in their precision and minuteness, a miracle in themselves. For Engineers Only Three pdf articles were recently written explaining the science of iridescence. Saito article Watanabe article Kinoshita article The text and pictures about iridescence can be seen in this doc file: The Science of Iridescence BugInABox.com searches the world for the finest in rare butterflies, beetles, and interesting insects, so you can enjoy them up close and personal. Copyright2004