Thin Film Interference
constructive/destructive interference - soap and oil films - reflection - phase shift
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 soap
films (suspended from a 19 cm diameter circular frame) or a very thin layer of oil
(floating on top of water). The soap film experiment will be described first.
(1) The soap film demonstration is a live replication of the experiment described and
pictured in PSSC Physics.
1
A soap film is made to span a circular plastic frame and is held vertically so that the
water slowly drains off the edge. Assume monochromatic illumination. Light that is
reflected from the front and back surfaces, towards the observer, is seen to undergo
constructive and destructive interference, depending on the thickness of the soap film as
well as the color of the light. 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. 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. The spacing of the light
and dark reflection bands across the soap film is an indication of how the thickness varies.
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 the previous demonstration
(Newton's Rings), the first maximum
of reflected light is essentially white and successive bands show more and more color.
(2) 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.
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.