More on Thin Film Interference
Interference of light waves explains a number of everyday observations, such as the colors in a soap bubble or in a thin film of oil floating on water. The colors are due to the constructive interference of waves reflected from the thin film- different colors interfere constructively for different viewing angles, leading to the rainbow effects you have no doubt observed.
Thin films are also used in optical components- sometimes to reduce or eliminate reflection (as in anti-reflective coatings on eyeglasses) or to produce a filter allowing only a band of wavelengths to pass. More expensive mirrors (not the bathroom medicine chest kind) are produced from multiple thin film technology.
To understand how a thin film works in a given application, we must look at
When solving these problems, it helps to take things one step at a time- see the example in the text (page81-83)
An important use of thin films is antireflective (AR) coatings applied to lenses and other optical elements. If you wear glasses with an AR coating, take them off and notice the reflection of a light source (bulb) in your glasses. Mine reflect green, which means they AR at either end of the visible spectrum. If you run water over your glasses, you will notice the reflected color changes (mine look purple-pink) as you form an "extra" layer of thin film with the running water.
Thin films of air, sometimes called "air wedges" can be used to inspect lenses and other surfaces for irregularities. When placed in contact with an "optical flat", a film of air forms between the two surfaces. Light reflecting from the surfaces of the air film form interference fringes that are straight if the surfaces are flat within a small fraction of the wavelength of light.
You can see this effect if you have access to thin glass pieces such as microscope slides. Before placing the slides one on top of the other, clean the surfaces between very carefully. (Isopropyl alcohol works well. Use a dust free tissue or paper towel.) Push the pieces together to squeeze out much of the air, and you might see a series of dark (or rainbow tinged) "fringes" due to the interference of light reflected from the inner (touching) surfaces of the glass. The fringes are easier to see if you use monochromatic light: reflect your laser pointer off of a waxed paper covered mirror onto the top glass surface. The waxed paper makes a nice diffuser for the laser light. The figure below shows the arrangement. (In this figure, a thin object- such as a hair or strip of plastic from a trash bag- has been inserted between the glass pieces at the right end to form an "air wedge".)