Thin Film Interference

What it shows:

Waves reflecting from two surfaces can interfere constructively and destructively. In this case it is light waves that are being reflected from the front and rear surfaces of thin soap or oil films. The interference produces a pattern of beautiful colors in white light, or dark and light bands in monochromatic light.

How it works:

Our two most visually dramatic illustrations of thin film interference use either a soap film suspended in air from a 19 cm diameter circular frame, or a very thin layer of oil floating on top of water.

Soap Film

The soap film demonstration is a live replication of the experiment described and pictured in Physical Science Study Committee Physics. 1

Wide view of reflected light from soap film, hung vertically.

Assume monochromatic illumination. Light reflected from the front and back surfaces of the film is seen to undergo constructive and destructive interference, depending on the thickness of the soap film. The reflection from the front surface is a so-called "hard reflection" and results in a 180° phase shift because the light is traveling from a medium of low refractive index (air) to a higher refractive medium (soap film). Going from soap-to-air, the reflection from the rear surface is a "soft reflection" and is not accompanied by phase reversal. Therefore, at the top of the soap film, where the thickness of the film is much less than the wavelength of light (and thus the path difference between the front and rear surface reflections is small compared to the wavelength) one observes no reflection (the film looks dark) due to the cancellation of the two waves.

Dramatic colors emerge from thin soap film reflection.

The soap film increases in thickness towards the bottom. Thus, reflections from the front and rear of the soap film gradually undergo increasingly larger phase shifts. For example, where the thickness is 1/4 wavelength, the path in the soap film is 1/2 wavelength longer than the front surface reflection (assuming normal incidence), putting the two reflections in phase and thereby giving a maximum reflected light intensity. As we continue to increase in thickness, every 1/4 wavelength increase in thickness we move from a region of no reflection to a region of maximum reflection to no reflection again, etc.

Bands of constructive interference become more tightly spaced as soap film thickness increases.

The situation is a little more complicated in white light, albeit much more beautiful. The reflection maxima occur when the thickness is (1/4)λ, (3/4) λ, (5/4) λ,... for a given color. As explained in (Newton's Rings), the first maximum of reflected light is essentially white and successive bands show more and more color.

Close up of reflected colors, includes white, yellow, blue, cyan, and magenta.

Sometimes the soap film will persist for several minutes.

Oil Film

For the oil film experiment, a drop of turpentine is squeezed out of an eyedropper onto the surface of water. The drop quickly spreads out into a thin film displaying rings of color which then quickly evolve into moving random patterns of brilliant colors. It's a beautiful animated version of the colorful oil slicks one sometimes sees in the street. We have measured the index of refraction of turpentine to be 1.52; thus, like the soap film experiment, light suffers a hard and soft reflection from the top and bottom surfaces, respectively.

soap films

Setting it up:

The light box, the holder for the soap-film frame, and the video camera all occupy a demo cart. Their relative positions should be carefully arranged before lecture as the lighting, viewpoint, and choice of lens is crucial. Position both camera and light box approximately 45° relative to the plane of the soap-film; the background behind the soap film should be as dark as possible. Choose the appropriate lens and camera distance so that the entire soap-film frame fills the monitor. 2 The pan of soap bubble solution also serves as the drip pan under the soap film holder.

Many recipes for soap bubble solutions have been published and all will do the job. However, the thicker solutions (typically the ones that have a lot of glycerine or even sugar) will not show the uniform banding pattern and tend to display more dynamic convection patterns and swirls. These are very beautiful but the thinner solutions will give the kind of interference pattern described above. We use a recipe from the Exploratorium which calls for 2/3 cup Dawn™ dishwashing soap, 2 to 3 tablespoons glycerine, and one gallon of water (let stand overnight). How long the soap film lasts on the frame before breaking depends very much on the relative humidity in the room - soap films don't last long when the relative humidity is below 50%. The more humid the better.

Comments:

If you would like to be a little more quantitative in the oil film experiment, you may wish to estimate the thickness of the film. This can be easily be done by measuring the volume of the drop (use a micro-pipet instead of an eyedropper) and the diameter of the circular oil slick.

1 Physical Science Study Committee, Physics, (D.C. Heath, 1960) Chapt. 19 sect. 9 "Interference in Thin Films." The color photographs of the soap films have been reproduced in many physics textbooks since.
2 The Canon 11.5-90 mm zoom lens has worked well in this application.