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Other examples
Interference of polarized white light in an optically anisotropic substance such as ice or crumpled cellophane, derived from double refraction, leads to these colors. The same effect in the form of photoelastic stress analysis is used to check glass for strains and to study the stresses in machinery and in medieval cathedrals in deformed plastic models.
Interference colors in a thinned ice cube,
3 cm across (between crossed polarizers).
Green metallic-like reflection of a photographic flash from the eyes of a cat, caused by multiple thin-film interference.
The eyes of many nocturnal animals contain multilayer structures that improve night vision and produce iridescent metallic-like reflections.
Several species of tropical marine predators, stomatopod crustaceans, have colored display surfaces that change dramatically when viewed with a linear polarizing filter. If the angle of polarization is changed, the uropods and antennal scales change from red-orange to faint purple. Researchers are studying the cause of this color (perhaps interference, perhaps a pigment), and whether the animals use these signals to communicate.
Views:
Scales of stomatopod crustaceans viewed with different angles of a polarizing filter. Photos taken a few moments apart.
Iridescence is caused by interference
Constructive vs. destructive interference. Light waves 1 and 2 produce constructive reinforcement if they are in phase (A) or destructive cancellation if they are out of phase (B).
Interference of light beams reflected from the front and back surfaces of a thin parallel film.
Light waves can interfere with one another. Most people observe some type of optical interference every day, but do not realize what is occurring to produce this phenomenon. The iridescent colors you see in a layer of oil on a wet pavement or in a soap bubble are produced by interference. So are the metallic colors found in some insects. These colors are produced by an object's surface structure, rather than by incorporated pigment molecules and they are often referred to as "structural colors."
Two light waves of the same wavelength can interact under appropriate circumstances so as to reinforce if they are in phase or cancel if they are out of phase, as shown in Fig. 33. In this section are discussed only those causes of color that involve interference without diffraction; the combination is covered in the following section.
The first clear demonstration of interference without the simultaneous occurrence of diffraction was performed about 1815 by the French scientist A. Fresnel. He used a monochromatic light source reflected in two mirrors; the mirrors were made of black glass to reflect light only at the front surface and were inclined at a small angle to each other to produce two overlapping beams of light on a screen. The result was a series of "interference fringes" consisting of alternate bands of light and dark. If either mirror was covered or removed, the fringes disappeared and only the uniform illumination derived from the other mirror remained.
The availability of monochromatic light from lasers has simplified the study of interference and has led to the wide use of a variety of interference-based devices. This includes Twyman-Green and multiple-reflection interferometers, such as Fabry- Perot etalons, used for precision measurements, as well as interference filters.
Consider a plane, coherent, monochromatic beam of light A-A incident at an angle onto a thin film such as a sheet of glass or plastic, as in Fig. 34. Part of wave B will enter the film as shown. Part of this beam will be reflected at the back surface at C and a part of this reflected beam will leave in direction D. Consider a second wave E in beam A-A, part of which is reflected at the upper surface so that it too leaves in direction D. As drawn, there is an extra path length of exactly five wavelengths while beam B traverses the distance 2b within the glass, as against the one wavelength a that beam E travels in the air. The net path difference is thus four wavelengths, so that the two beams might be expected to be exactly in phase with each other. However, reflection at a medium of higher refractive index as at the top surface produces a phase change equivalent to one-half wavelength, whereas this does not happen at the lower surface, which is reflection at a medium of lower refractive index. Accordingly, the two beams appearing in direction D are out of phase as shown and will undergo destructive cancellation as at B in Fig. 33. As either the angle, the thickness, or the wavelength changes, alternate dark regions from cancellation and light regions from reinforcement will occur.
In a tapered film with monochromatic light a series of dark and light bands occurs, while with white light the sequence of overlapping light and dark bands from the spectral colors leads to "Newton's colors." Starting with the thinnest film this sequence is black, gray, white, yellow, orange, red (end of the first "order"), violet, blue, green, yellow, orange-red, violet (end of the second "order"), blue, green, yellow, red, and so on. Newton's colors are seen in the tapered air gap between touching non-flat sheets of glass, in cracks in glass or in crystals, in a soap bubble, in an oil slick on a water surface, and in the petrological microscope.
Antireflection coatings, as on camera lenses, employ this effect. A coating that has the geometrical mean value of the refractive index intermediate between those of glass and air and a thickness of one quarter the wavelength of light can reduce the overall reflected light to less than one half, while multiple layers can reduce this to less than one tenth; such a layer usually appears purple to the eye.
There are a large number of structural colorations in biological systems that are derived from thin-film interference, usually in multiple-layered structures. The layers may be composed of keratin, chitin, calcium carbonate, mucus, and so on, and are frequently backed by a dark layer of melanin, which intensifies the color by absorbing the nonreflected light. Such biological interference colorations are usually "iridescent," this designation implying that multiple colors as in the rainbow (Latin iris) are seen, and also that the colors change with the orientation. Examples include pearl and mother of pearl, the transparent wings of house and dragon flies, and iridescent scales on beetles and butterflies, as in Plate XII, and on the feathers of hummingbirds and peacocks.
