In physics, a new waveform results when two or more waves superimpose on each other, and this development is referred to as interference. This interference, as it applies to waves, is either constructive or destructive, or a combination of both. In constructive interference, “the amplitude of the wave that results is greater than that of the original waves” (Hecht 87).
This occurrence is in contrast to destructive interference whereby the amplitude of the resultant wave is lesser than that of the original. Light is made up of waves, obeys all the rules of waves, and is thus subject to superimposition, and hence interference.
For interference to occur, some conditions that relate to the source of light or waves should be taken into consideration. There should be coherence of the sources, that is, they must maintain a constant phase with respect to each other. The sources should also be monochromatic meaning that they have a single wavelength.
A commonly used method to produce coherent sources is by using a single source of light and illuminating a barrier with two slits. The light emerging from these slits is, thus coherent. The waves spread out from the slits according to Huygens principle, and this divergence of light from the initial line of travel is what is called diffraction (Gordon, Beichner, and Serway 23).
Thomas Young first demonstrated the principle of interference in light waves from two sources in 1801, where two slits acted as sources of the light waves. The waves were always in phase since they were from the same wave front. The light passed through the slits and illuminated a screen.
A stationary interference pattern occurred on the screen. Constructive interference occurred where dark fringes resulted while destructive interference occurred where bright fringes occurred (Jenkins and Harvey 65). This case is an outstanding example of interference of light from a coherent source.
The phenomenon of interference, depending on the mode of production, has traditionally been divided into two classes. In the first class, the division of the wave front into two parts produces the interference by use of the phenomenon of diffraction, refraction and reflection (Fowles 89).
Young’s double slit experiment falls under this form of interference. Amplitude division of the incident light produces the second nature of interference. This occurs by either parallel reflection or refraction of the incident light. The resulting light waves reinforce each other after covering different distances producing interference. An example of this form of interference is Newton’s ring.
The phenomenon of interference can explain the colors commonly seen on soap bubbles, oil slicks or even thin films. In all the above examples, interference pattern formation is by amplitude division. In the thin film observation, for example, plane waves fall on it, and light waves reflected from the lower and upper surfaces interfere with each other.
Since the condition of interference is influenced by the thickness of the film, angle of refraction and the wavelength, the eye observes different colors at different positions. Other colors will be absent where an only one-color maximum is satisfied, and hence only this color will be seen at this position (Knittl 43).
By using the same principle, Newton’s ring becomes easy to understand. If a Plano-convex lens is placed on a glass plate with its convex surface, a film of air with a gradually increasing thickness is formed in between the two. Alternating dark and bright circular fringes are seen when monochromatic light falls normally.
The fringes appear “circular because the air film has circular symmetry” (Jenkins and Harvey 49). The Newton’s rings are formed because of “the interference of the reflected waves from the top and bottom surfaces of the air films between the plates” (Knittl 46).
For the soap bubble, “light traveling through air strikes the soap film” (Bass 87). Air with has a larger refractive index than the film. Refraction occurs at the upper film surface, and transmission to the lower surface occurs interfering with the other waves. This creates the observed patter (Bass 88).
The soap bubble thus appears lovely with colors of the rainbow due to this phenomenon. The striking resemblance of the color patterns observed in the oil film and soap bubble only serves to indicate the similarity in the formation process of the two.
There are many similar applications of this phenomenon of interference. Some animals we consider beautiful with iridescent colors apply this principle. An example is the Morpho didius butterfly, which commonly inhabits the Amazon rainforest and can be found flying high on a normal day.
It appears bright blue due to the natural grating on its wings. Most people would think that it is due to a dye. Another animal considered being among the most beautiful and attractive is the peacock with its colorful tail. It applies the principle of interference of colors that it acquired naturally to produce the different colors observed on its tail. Pearl shells and opals also utilize this principle of interference of light and colors for camouflage and beauty and is an eminent character in their process of finding a mate.
One or more colored light rings are usually seen around the moon when it shines through light clouds. This occurrence is due to the light from the moon diffracting as it penetrates the water and ice droplets in the light clouds.
These haloes seen around the moon also appear around streetlights on foggy or misty nights and are all because of the principle of diffraction and interference (Gordon, Beichner, and Serway 75). The colors appear beautiful and are because of the many wavelengths in light. Another example is the hazy appearance of smog.
Light passing through the smog particles is diffracted, scattered and absorbed producing the hazy appearance (Knittl 67). Research around this property has resulted to highly innovative inventions applied in some areas, around the world, to establish the cleanliness of air and water turbidity. This has contributed in efforts of environmental health and assessment of levels of pollution especially in the major cities of the world enabling proper environmental rehabilitation measures.
Holograms, like those seen on credit cards, for example, diffract each color from a different angle creating a complicated pattern of lines on the card. This behavior is utilized or security purposes. Diffraction is applied to measure exceptionally small distances, and diffraction grating is applied, in spectroscopes, to investigate the color component of light from specified sources. In diffraction grating, each color of light diffracts, at a specified angle, producing the various colors.
Thin films have the commercial use in mirrors, optical fibers and anti-reflection coatings as well as other optical materials. For a given wavelength, “thin films are in the market engineered to control the amount of light transmitted or reflected through a surface” (Jenkins and Harvey 35). A Fabry-Perot etalon utilizes “the principle of the thin film interference to select the wavelengths of light transmitted through this device” (Bass 47).
A special application of the above properties of light is in interferometry, which is the science, and art of using coherent light to make measurements. When interference of light is measured, then the distance it has covered is easily established. Some of the applications of interferometry are optical testing, which is the use of interferometry to measure surface quality and inspection of slip gauges and measurement standards.
Another application in interferometry is direct phase measurements in multiple wavelengths and phase stepping and phase shifting. Another use is in the alignment of unusually high quality lenses such as those used in telescopes, cameras, and steppers, which are photolithographic tool used in fabricating intricate circuit patterns.
Another use is to measure small angular sizes from distant stars. The oldest form of interferometer that is used is the Michelson interferometer, but which has been modified with the introduction of sophistication (Fowls 56).
In conclusion, the interference of colors, which is due to interference of light, results in the production of wavelengths, which are different from incident light. This principle of waves has many applications in nature as seen above and science is in the forefront in the application of this phenomenon. Advances in the field of interferometry applied this principle, and is a fundamental branch of science. A lot of research still needs to go into this branch of science to maximize on the principle, which is not fully exploited.
Bass, Michael. Handbook of Optics. 2nd Ed. New York: McGraw-Hill, 2002. Print.
Fowles, Grant. Introduction to Modern Optics. 2nd Ed. New York: Dover Publications, 1975. Print.
Gordon, John, Robert Beichner, and Raymond Serway. Student Solutions Manual & Study Guide to Accompany Physics for Scientists and Engineers. 5th ed. Fort Worth: Harcourt College Pub, 2000. Print.
Hecht, Eugene. Optics. 4th ed. Boston, MA: Addison-Wesley, 2002. Print.
Jenkins, Francis, and Elliott Harvey. Fundamentals of Optics. 3rd Ed. New York: McGraw-Hill, 1965. Print.
Knittl, Zdenek. Optics of Thin Films: An Optical Multilayer Theory. London: Wiley, 1976. Print.